requirements: Update Django requirements
requirements: Update various requirements versions

Also drop exact versions for travis as we will always want the latest ones.
requirements: Update Raven optional dependency to 5.x
Travis: Always use latest packages from pypi
Travis: Use system site packages to avoid recompiling lxml
Travis: Use travis_retry for installing things
Travis: Cache apt packages
settings: Remove the INTERCEPT_REDIRECT option from dev settings

The option is deprecated on newer versions of django-debug-toolbar