Auto merge of #4944 - dekellum:missing-require-rubygems-source, r=ind…

…irect

[RubygemsExt] Add missing require rubygems/source

The change referenced below as released in 1.13.0.rc.2, may attempt to reference Gem::Source without it being loaded, resulting in:

~~~
  [!] There was an error parsing `Gemfile`:
  [!] There was an error while loading `elided.gemspec`: uninitialized constant Gem::Source. Bundler cannot continue.
~~~

Observed this on ruby 2.2.5 with stock rubygems 2.4.5 as well as upgraded rubygems 2.6.6. Add this require.

f9de70e  by @segiddins:
> [RubygemsExt] return Source::Installed from #source when appropriate

lib/bundler/rubygems_ext.rb

@@ -8,6 +8,16 @@
 
 require "rubygems"
 require "rubygems/specification"
+
+begin
+  # Possible use in Gem::Specification#source below and require
+  # shouldn't be deferred.
+  require "rubygems/source"
+rescue LoadError
+  # Not available before Rubygems 2.0.0, ignore
+  nil
+end
+
 require "bundler/match_platform"
 
 module Gem