Update to django 1.7c1 in .travis.yml
Add check for VerifyEmailManager

* A User model subclassing VerifyEmailMixin should have a manager
  subclassing VerifyEmailManager
Don't catch ImportError for checks module

* The checks module exists for django >= 1.6
* Add skip_if_checks_unavailable decorator to avoid repetition
Merge pull request #59 from incuna/checks

Add check for misconfigured User model