Deprecate Mustang runtime

Gemfile

@@ -6,9 +6,6 @@ group :test do
   gem 'json'
   # see https://github.com/jbarnette/johnson/issues/21
   gem 'johnson',      :platform => :mri_18
-  # see https://github.com/nu7hatch/mustang/issues/18
-  gem 'mustang',      :platform => :ruby,
-    :git => "https://github.com/nu7hatch/mustang.git", :ref => "2a3bcfbd9fd0f34e9b004fcd92188f326b40ec2a"
   # disabled for rbx, because of https://github.com/cowboyd/therubyracer/issues/157
   gem 'therubyracer', :platform => :mri
   gem 'therubyrhino', ">=1.73.3", :platform => :jruby

README.md

@@ -13,8 +13,6 @@ ExecJS supports these runtimes:
   Rhino embedded within JRuby
 * [Johnson](https://github.com/jbarnette/johnson) - Mozilla
   SpiderMonkey embedded within Ruby
-* [Mustang](https://github.com/nu7hatch/mustang) - Mustang V8
-  embedded within Ruby
 * [Node.js](http://nodejs.org/)
 * Apple JavaScriptCore - Included with Mac OS X
 * [Microsoft Windows Script Host](http://msdn.microsoft.com/en-us/library/9bbdkx3k.aspx) (JScript)

lib/execjs/mustang_runtime.rb

@@ -68,5 +68,9 @@ module ExecJS
     rescue LoadError
       false
     end
+
+    def deprecated?
+      true
+    end
   end
 end