Remove unnecessary version guard from versions.rb

Removing it triggers the following warnings when running `bundle` under
jruby from the root of the `psych` repo prints the following warnings:

```
/path/to/jruby-9.2.9.0/lib/ruby/gems/shared/gems/psych-3.1.0-java/lib/psych/versions.rb:7: warning: already initialized constant VERSION
/path/to//jruby-9.2.9.0/lib/ruby/gems/shared/gems/psych-3.1.0-java/lib/psych/versions.rb:10: warning: already initialized constant DEFAULT_SNAKEYAML_VERSION
```

This is because bundler loads the versions file relatively from the
local gemspec, and then internally loads the psych gem, causing the
redefinition warnings.

Instead, we modify the $LOAD_PATH so that when working locally on the
`psych` repo, the local version of `psych` gets used.

lib/psych/versions.rb

@@ -2,7 +2,7 @@
 # frozen_string_literal: true
 module Psych
   # The version of Psych you are using
-  VERSION = '3.1.0' unless defined?(::Psych::VERSION)
+  VERSION = '3.1.0'
 
   if RUBY_ENGINE == 'jruby'
     DEFAULT_SNAKEYAML_VERSION = '1.23'.freeze

psych.gemspec

@@ -1,12 +1,10 @@
 # -*- encoding: utf-8 -*-
 # frozen_string_literal: true
 
-begin
-  require_relative 'lib/psych/versions'
-rescue LoadError
-  # for Ruby core repository
-  require_relative 'versions'
-end
+lib_path = File.expand_path('lib', __dir__)
+$LOAD_PATH.unshift lib_path if File.exist?(lib_path)
+
+require 'psych/versions'
 
 Gem::Specification.new do |s|
   s.name = "psych"