Do not install ffi gem automatically on Windows platforms

We originally introduced this logic in #132 without understanding all
the implications.

Issue #158 raised a challenge with this approach.

Gemfile

@@ -3,6 +3,9 @@ source 'http://rubygems.org'
 # Specify your gem's dependencies in child_process.gemspec
 gemspec
 
+# Used for local development/testing only
+gem 'rake'
+
 if RUBY_VERSION =~ /^1\./
   gem 'tins', '< 1.7' # The 'tins' gem requires Ruby 2.x on/after this version
   gem 'json', '< 2.0' # The 'json' gem drops pre-Ruby 2.x support on/after this version

README.md

@@ -16,6 +16,8 @@ a standalone library.
 
 * Ruby 2.3+, JRuby 9+
 
+Windows users **must** ensure the `ffi` gem (`>= 1.0.11`) is installed in order to use ChildProcess.
+
 # Usage
 
 The object returned from `ChildProcess.build` will implement `ChildProcess::AbstractProcess`.

childprocess.gemspec

@@ -23,9 +23,4 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rspec", "~> 3.0"
   s.add_development_dependency "yard", "~> 0.0"
   s.add_development_dependency 'coveralls', '< 1.0'
-
-  s.add_runtime_dependency 'rake', '< 13.0'
-
-  # Install FFI gem if we're running on Windows
-  s.extensions = 'ext/mkrf_conf.rb'
 end