sass-rails 5.0 support

* Based upon #200 and some squashed commits from #197 by @danielpclark.
* Now includes a working test suite for Travis-CI
* New tests that cover asset precompilation scenarios. This should help with future
  regressions (in theory).

Closes #198
Closes #195
Closes #197
Closes #194
Closes #191
Closes #192

/cc @danielpclark, @Intrepidd, @hsbt