Update ember-test-helpers minimum version.
Update ember-test-helpers minimum version.

Allows mocking factories in integration tests.

SaladFork/ember-mocha,emberjs/ember-mocha,SaladFork/ember-mocha,switchfly/ember-mocha,emberjs/ember-mocha,switchfly/ember-mocha,cowboyd/ember-mocha,switchfly/ember-mocha,alexgb/ember-mocha,SaladFork/ember-mocha,cowboyd/ember-mocha,cowboyd/ember-mocha,alexgb/ember-mocha,alexgb/ember-mocha