Perform gem uninstallation for prune

plugins/commands/plugin/action/prune_gems.rb

@@ -1,4 +1,5 @@
 require "rubygems"
+require "rubygems/uninstaller"
 require "set"
 
 require "log4r"
@@ -121,7 +122,29 @@ module VagrantPlugins
           prune_specs = all_specs - good_specs
           @logger.info("Gems to prune: #{prune_specs.inspect}")
 
-          # TODO: Prune
+          if prune_specs.length > 0
+            env[:gem_helper].with_environment do
+              prune_specs.each do |prune_spec|
+                uninstaller = Gem::Uninstaller.new(prune_spec.name, {
+                  :executables => true,
+                  :force       => true,
+                  :version     => prune_spec.version.version
+                })
+
+                # This is sad, but there is no way to force this to be true
+                # so I just monkey-patch here. Vagrant has a pretty strict
+                # version check on the RubyGems version so this should be okay.
+                # In the future, let's try to get a pull request into RubyGems
+                # to fix this.
+                def uninstaller.ask_if_ok(spec)
+                  true
+                end
+
+                @logger.info("Uninstalling: #{prune_spec.name} (#{prune_spec.version})")
+                uninstaller.uninstall
+              end
+            end
+          end
 
           @app.call(env)
         end