Added tests for ruby/compass/gulp and made sure both apps can access `which`

The original design didn't have which in both package.json files. A test was
added to protect against this case. Additionally, logic was added to test
that the command line utilities needed were available. This wrap-up message was
refactored down into slideshow-base.

closes #1
Merge remote-tracking branch 'origin/master'

* origin/master:
  Update README.md