Re-write specs to use more things

* use parslet's rspec matcher (`expect(parser).to parse "text"`)
* use rspec `after(:each)` to keep tedious repetition fairly succinct
* test the parslet parser object, not my encapsulating Smashcut object
* use pre-release parslet 1.6 for less deprecation warnings

close #9
Merge pull request #10 from maxjacobson/test-the-parser

Re-write specs to use more things