Use ember-blueprint-test-helpers for testing axe version

[Brian Sipple]

This was brought up in #38, but it would be better to use ember-blueprint-test-helpers to verify that the version of axe-core that we're adding to projects is in sync with our current dependency.

At the moment, however, this still isn't possible

[Trent Willis]

This is no longer needed as we don't have a blueprint (due to moving axe-core to npm).