Always require spec_helper for RSpec runs

Saves having that require statement in every file.
Use `falsey` and `truthy` because RSpec 3.x

be_false and be_true are gone.
Install Guard and plugins
Add workaround for legacy RSpec matcher warnings
Upgrade .should matchers to new expectation style

`should` matchers are deprecated. We'll just take the once-off hit and
fix this.
Change mocks to use new style; stub is deprecated