Bump django dependency to 1.6

This change updates our django dependency to >= 1.6.1.

With the previous change to make sure that initialize() is called from test
setUp() methods, this is pretty small:
* Update the hg and bzr tests to properly detect missing dependencies and skip
  tests as appropriate.
* Remove the django.contrib.markup app. We got rid of our usage of this a while
  ago and forgot to remove it from INSTALLED_APPS. In Django 1.6 it's been
  removed entirely.
* Change the setup.py check to reject Python 2.6 versions less than 2.6.5.
* Fix call to compile_messages() in build-i18n.py

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5310/