Add google/google-api-php-client @2.0.0 compatibillity

[JustinElst]

This removes the other authentication option. And is just a quick "getting it to work" patch.
It should be tested, and people should be informed that they need to do the necessary changes in there config.

[pquerner]

Why does it remove the P12 authentification?

[JustinElst]

Because p12 is being deprecated:
https://github.com/google/google-api-php-client/blob/998067619ce59a849948b0a8647499b90f70ab41/UPGRADING.md#user-content-google_auth_assertioncredentials-has-been-removed

Note: P12s are deprecated in favor of service account JSON, which can be generated in the Credentials section of Google Developer Console.

[rvelhote]

@MrGekko I added PR #94 which works with the most recent version of GoogleShoppingAPI (your PR is quite old with merge conflicts). I also attempted to maintain backwards compatibility for older Google API Client libraries.