Remove posix runner and unsupported posix-spawn gem

Gemfile

@@ -1,7 +1,3 @@
 source 'https://rubygems.org'
 
 gemspec
-
-platforms :ruby do
-  gem "posix-spawn"
-end

README.md

@@ -145,23 +145,22 @@ second argument of `new`. Terrapin assumes that you will not be manually
 passing user-generated data to that argument and will be using it as a template
 for your command line's structure.
 
-## POSIX Spawn
+## Runners
 
-You can potentially increase performance by installing [the posix-spawn
-gem](https://rubygems.org/gems/posix-spawn). This gem can keep your
-application's heap from being copied when forking command line
-processes. For applications with large heaps the gain can be
-significant. To include `posix-spawn`, simply add it to your `Gemfile` or,
-if you don't use bundler, install the gem.
+Terrapin will choose from among a couple different ways of running commands.
+The simplest is `Process.spawn`, which is also the default. Terrapin can also just use [backticks], so if for some reason you'd prefer that, you can ask Terrapin to use that:
 
-## Runners
+```ruby
+Terrapin::CommandLine.runner = Terrapin::CommandLine::BackticksRunner.new
+```
 
-Terrapin will attempt to choose from among 3 different ways of running commands.
-The simplest is using backticks, and is the default in 1.8. In Ruby 1.9, it
-will attempt to use `Process.spawn`. And, as mentioned above, if the
-`posix-spawn` gem is installed, it will attempt to use that. If for some reason
-one of the `.spawn` runners don't work for you, you can override them manually
-by setting a new runner, like so:
+And if you really want to, you can define your own Runner, though I can't imagine why you would.
+
+[backticks]: https://ruby-doc.org/3.2.1/Kernel.html#method-i-60
+
+Terrapin::CommandLine.runner = Terrapin::CommandLine::BackticksRunner.new
+And if you really want to, you can define your own Runner, though I can't
+imagine why you would.
 
 ```ruby
 Terrapin::CommandLine.runner = Terrapin::CommandLine::BackticksRunner.new

lib/terrapin/command_line.rb

@@ -38,7 +38,7 @@ def unfake!
       private
 
       def best_runner
-        [PosixRunner, ProcessRunner, BackticksRunner].detect do |runner|
+        [ProcessRunner, BackticksRunner].detect do |runner|
           runner.supported?
         end.new
       end

lib/terrapin/command_line/runners.rb

@@ -2,6 +2,5 @@
 
 require 'terrapin/command_line/runners/backticks_runner'
 require 'terrapin/command_line/runners/process_runner'
-require 'terrapin/command_line/runners/posix_runner'
 require 'terrapin/command_line/runners/popen_runner'
 require 'terrapin/command_line/runners/fake_runner'

spec/terrapin/runners_spec.rb

@@ -3,7 +3,6 @@
 describe "When picking a Runner" do
   it "uses the BackticksRunner by default" do
     Terrapin::CommandLine::ProcessRunner.stubs(:supported?).returns(false)
-    Terrapin::CommandLine::PosixRunner.stubs(:supported?).returns(false)
 
     cmd = Terrapin::CommandLine.new("echo", "hello")
 
@@ -12,34 +11,11 @@
 
   it "uses the ProcessRunner on 1.9 and it's available" do
     Terrapin::CommandLine::ProcessRunner.stubs(:supported?).returns(true)
-    Terrapin::CommandLine::PosixRunner.stubs(:supported?).returns(false)
 
     cmd = Terrapin::CommandLine.new("echo", "hello")
     expect(cmd.runner.class).to eq(Terrapin::CommandLine::ProcessRunner)
   end
 
-  it "uses the PosixRunner if the PosixRunner is available" do
-    Terrapin::CommandLine::PosixRunner.stubs(:supported?).returns(true)
-
-    cmd = Terrapin::CommandLine.new("echo", "hello")
-    expect(cmd.runner.class).to eq(Terrapin::CommandLine::PosixRunner)
-  end
-
-  it "uses the BackticksRunner if the PosixRunner is available, but we told it to use Backticks all the time" do
-    Terrapin::CommandLine::PosixRunner.stubs(:supported?).returns(true)
-    Terrapin::CommandLine.runner = Terrapin::CommandLine::BackticksRunner.new
-
-    cmd = Terrapin::CommandLine.new("echo", "hello")
-    expect(cmd.runner.class).to eq(Terrapin::CommandLine::BackticksRunner)
-  end
-
-  it "uses the BackticksRunner if the PosixRunner is available, but we told it to use Backticks" do
-    Terrapin::CommandLine::PosixRunner.stubs(:supported?).returns(true)
-
-    cmd = Terrapin::CommandLine.new("echo", "hello", :runner => Terrapin::CommandLine::BackticksRunner.new)
-    expect(cmd.runner.class).to eq(Terrapin::CommandLine::BackticksRunner)
-  end
-
   it "can go into 'Fake' mode" do
     Terrapin::CommandLine.fake!
 
@@ -78,7 +54,6 @@
   [
     Terrapin::CommandLine::BackticksRunner,
     Terrapin::CommandLine::PopenRunner,
-    Terrapin::CommandLine::PosixRunner,
     Terrapin::CommandLine::ProcessRunner
   ].each do |runner_class|
     if runner_class.supported?