Eliminate hard runtime dependencies

[ajsharp]

From the gemspec:

  s.add_runtime_dependency 'sprockets',       '~> 2.0.0'
  s.add_runtime_dependency 'uglifier'
  s.add_runtime_dependency 'closure-compiler'
  s.add_runtime_dependency 'yui-compressor'
  s.add_runtime_dependency 'execjs'
  s.add_runtime_dependency 'therubyracer'

Sprockets itself doesn't force any of these dependencies on you, so I don't think sinatra-sprockets should either. We just want to use this to process and serve a few scss stylesheets, and don't want all the javascript runtime stuff, especially therubyracer which is a major pain to package in production.

So I'd advocate for the only runtime dependency here being sprockets itself, and if you want to use these other libraries, you need to include them in your Gemfile.