Semantically minor stuff: Gem updates and special-case cleanup.

Teaspoon 0.8 is out, but requires deceptively significant change from 0.7.x; since we're not *doing* any Script on this project (yet, at least), I decided to lock us at 0.7.9 until it becomes a problem.

Additionally,
* We're now developing and testing on multiple systems (a Linux VM and a MacBook Pro without Growl installed); leaving Bullet's Growl notifications in was causing problems. Hence, the development environment changes;
* The `ExplainErrors` helper was recycled from a project that needed to support two different "error objects"; this one isn't there yet, so commenting that out and leaving a single path brings us back to 100% coverage.

RSpec: 112 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 40 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1