Add version.rb to gemspec files
Add version.rb to gemspec files

When calling `require "sprockets/es6"` you get the following error:
`LoadError: cannot load such file -- sprockets/es6/version` because the
version isn't being packaged with the gem.

louisjimenez/sprockets-es6,blasterpal/sprockets-es6,mulvaney/sprockets-es6,Valve/sprockets-es6,TannerRogalsky/sprockets-es6,zackp30/sprockets-es6,rstacruz/sprockets-es6