Switch to explicitly declaring build matrix, with the versions
of Python supported by each version of Django we handle.
(Exclusion list was becoming too complicated.)
The build now tests for newer Python 3.x versions and PyPy.
Also the build will now test itself against Django 1.7RC1 instead of the latest beta version.
Test Django 1.2 and 1.3; Python 2.6 and 2.7
Don't bother testing Python 2.5 -- it requires an older version of
requests (that doesn't depend on json).
Don't bother testing Django 1.2 -- it requires changes to ``assertRaises``
in the test cases (because the context-manager version of assertRaises is
part of unittest2, which appears in Django 1.3+ or Python 2.7+).
Don't bother testing Django 1.5 (yet).
