Update dependencies and imports to work with EmberJS 3.24

[dfriquet]

Hello there!

While upgrading my app to EmberJS 3.24, my test suite started to fail with this error message :

Could not find module ember-test-helpers imported from ember-cli-pact/index

I noticed FreshBooks fork and a npm install --save https://github.com/freshbooks/ember-cli-pact/tarball/master solved everything.

Allow me to create this pull request to backport these effective changes if you will.

Best regards, David.