Clean RUBYLIB as well in with_clean_env

This still needs tests to ensure that `with_clean_env` does the right
thing.

refs #3982

lib/bundler.rb

@@ -236,12 +236,19 @@ def with_original_env
 
     def with_clean_env
       with_original_env do
-        ENV['MANPATH'] = ENV['BUNDLE_ORIG_MANPATH']
-        ENV.delete_if { |k,_| k[0,7] == 'BUNDLE_' }
-        if ENV.has_key? 'RUBYOPT'
-          ENV['RUBYOPT'] = ENV['RUBYOPT'].sub '-rbundler/setup', ''
-          ENV['RUBYOPT'] = ENV['RUBYOPT'].sub "-I#{File.expand_path('..', __FILE__)}", ''
+        ENV["MANPATH"] = ENV["BUNDLE_ORIG_MANPATH"]
+        ENV.delete_if { |k,_| k[0,7] == "BUNDLE_" }
+
+        if ENV.has_key?("RUBYOPT")
+          ENV["RUBYOPT"] = ENV["RUBYOPT"].sub "-rbundler/setup", ""
+        end
+
+        if ENV.has_key?("RUBYLIB")
+          rubylib = ENV["RUBYLIB"].split(File::PATH_SEPARATOR)
+          rubylib.delete(File.expand_path("..", __FILE__))
+          ENV["RUBYLIB"] = rubylib.join(File::PATH_SEPARATOR)
         end
+
         yield
       end
     end