Make last_page? behave consistently with last?

Closes #582.

Also amends next_page and helpers for action_view and sinatra to
preserve the intended behaviour.
Update CONTRIBUTING.md [ci skip]
Merge pull request #583 from lime/last-page-consistency

Make last_page? behave consistently with last?
Raise ZeroPerPageOperation when total_pages/current_page is called after per(0)

closes #296, closes #585
Add 4.2.0.beta1 to tested Rails versions [ci skip]

Skip tests for now, we can run them on next change.
Run test for AR 4.2 on travis
Do not run tests for AR 4.2 against Ruby1.8