Replace deprecated approval_prompt with prompt

[venkatd]

approval_prompt has been deprecated and no longer works. It has been replaced by the prompt option. You can see the list of parameters here:
https://developers.google.com/identity/protocols/OAuth2WebServer#creatingclient

In the future, it may be worth considering an alternate strategy of just passing through all options directly so a code update isn't necessary every time the options change.

Resolves #49 and is the same as #50 except it renames prompt to approval_prompt because the later is no longer honored by Google's OAuth.

[venkatd]

Looks like travis doesn't recognize this as an elixir project. I can include a travis.yml if desired.

[doomspork]

@venkatd if you're interested in contributions a .travis.yml would be greatly appreciated. In fact, we need one for every provider project 😰

[pyromaniac]

This is awesome, thanks for the follow-up! Any chance for the next release to be done?