Fix source map issue(#212)

We should not specify '-j' option for compiling to JavaScript.
It should be used only Rails '.js.coffee' file for avoiding
'.js.js' file generated.

If you want to generate source map file in Rails project,
please see following project.

  https://github.com/markbates/coffee-rails-source-maps
Merge pull request #213 from defunkt/fix-sourcemap-issue

Fix source map issue(#212)