Update build-source-gem script to support v2.

[gobijan]

As far as I understood the coffee-script integration for rails depends on coffee-script-source gem which is built via this script.
I modified the script to use version 2 of the CoffeeScript compiler instead of version 1.
Also I make sure that if no version is set I get the latest tag for the build.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @schneems (or someone else) soon.

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[rafaelfranca]

This also needs to keep supporting the version 1.

[MatrixFr]

@gobijan can you explain how to use it and specify V2 in rails app ? (who using "gem 'coffee-rails'" only, by default when create new rails app and not "ruby-coffee-script"). What we have to added to gem file?

[ghost]

@Argorate After investigation, I made coffeescript2 worked with sprockets together, hope this answer helps .

1. Install latest coffee-script gem.
2. Add COFFEESCRIPT_SOURCE_PATH to your operation system's variables.
3. Set COFFEESCRIPT_SOURCE_PATH value to your coffeescript.js path.
   Please make sure you use a standalone file, you can get it from https://coffeescript.org/v2/browser-compiler/coffeescript.js.

Everything done, enjoy!

FYI https://github.com/rails/ruby-coffee-script#installation