Remove duplicate compass-rails gem declaration

There is already a gemspec declaration, so there's no need for an extra
compass-rails delclaration in the Gemfile.
Switch to recommended Travis CI integration for appraisal

See https://github.com/thoughtbot/appraisal#travis-ci-integration
Removed bundler caching in travis.yml

Bundler caching adversely affects the nature of the test suite
Fixed test suite
Simplified rails version arguments throughout test suite
Support new SassImporter method signature

Fixes #195
Updated CacheStore
Files will update when changes are made now.
Added rails 4.2 and sass-rails 5.0 to test suite
Added tests to cover assets:precompile scenarios
Merge pull request #201 from rmm5t/sass-rails-5.0

sass-rails 5.0 support