Locking Thor to 0.18.1 to fix the CLI

Two changes here:

1) Locking Thor to version 0.18.1 (back from 0.19.1). This will resolve the problem where it kept asking for input during the testing. I tracked the issue down to a new incompatibility between Thor and Mocha whereby Mocha can't read from $stdout anymore. The only fix/hack I've found so far is [here](http://stackoverflow.com/questions/23007416/mocha-expect-failing-with-thor-0-19-1) but requires a patch and a refactor of the entire CLI test suite. Sticking Thor to 0.18.1 seems like the most reasonable short term solution.

2) Require yaml in cli.rb. This is just something it needs now, it seems

@pickle27