Move spoon dep to Gemfile

By moving it to the Gemfile & using platform we're able avoid installing
spoon for other Ruby implementations.

Gemfile

@@ -2,6 +2,10 @@ source "http://rubygems.org"
 
 gemspec
 
+platform :jruby do
+  gem 'spoon', '~> 0.0.1'
+end
+
 group :development do
   gem 'parka'
   gem 'rake'