Green specs again

RSpec 3.0 has broken a few things in the specs. This fixes the things that were now causing actual errors, hides the deprecation warnings for things that are merely deprecated, and broadens the range of Rubies that Travis tests against to include more modern releases.

 * `be_true` and `be_false` are now `be_truthy` and `be_falsy`.
 * `:example_group` is now deprecated.
 * `should` syntax is deprecated in favor of `expect`-style assertions. I just enabled the "should" syntax, for the moment at least.
 * I've pinned rspec and rspec-given to the 3.x line in the `Gemfile` to avoid future surprises.