Fix Bundler 1.8.1+ test failures.
Fix Bundler 1.8.1+ test failures.

This fixes issues such as:

~~~
  1) BundlerExt::Runtime add_spec adds spec load paths not already on LOAD_PATH to it
     Failure/Error: runtime.add_spec(spec)
     LoadError:
       cannot load such file -- rubygems/ext/builder
     # /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:83:in `require'
     # /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:83:in `require'
     # ./lib/bundler_ext/runtime.rb:20:in `new'
     # ./lib/bundler_ext/runtime.rb:20:in `rubygems'
     # ./lib/bundler_ext/runtime.rb:64:in `add_spec'
     # ./spec/bundler_ext/runtime_spec.rb:164:in `block (3 levels) in <module:BundlerExt>'
     # ./spec/bundler_ext/runtime_spec.rb:145:in `block (3 levels) in <module:BundlerExt>'
~~~

These are caused by Bundler requiring to load additional libraries [[1]]
since Bundler 1.8.1.

[1]: https://github.com/rubygems/bundler/commit/f86bd0712c3a1ed8138e5c0a056d6a5511e207c9

bundlerext/bundler_ext