Use user.is_authenticated method if possible

[bbayles]

This PR changes axes.decorators.check_request such that it uses the User.is_authenticated method when running under Django 1.10 and above.

The callable is_authenticated() method was deprecated in Django 1.10 - see the notice here. This PR prevents this warning from showing up:

RemovedInDjango20Warning: Using user.is_authenticated() and user.is_anonymous() as a method is deprecated. Remove the parentheses to use it as an attribute.

[coveralls]

Coverage increased (+0.05%) to 91.691% when pulling f179b1d on bbayles:is_authenticated_method into 87731d1 on jazzband:master.

[camilonova]

Thank you. At the new version this is no longer needed.