Python 3 support, Travis CI & a few modernisations

In order to achieve [johnny-cache 1.5 goals](http://packages.python.org/johnny-cache/#deprecation-policy), I made the following:
- I added Travis CI to smoothly move from 1.4 to 1.5, but discovered that **johnny-cache was not able to run with Django 1.1** (and Python 2.4 & 2.5 are not available on Travis CI)
- so I wanted to speed things a bit. I implemented full Python 3 support and decided to drop Django<1.3 & Python<2.6 support (as specified in the 1.5 goals).
- in tests, I changed old `failUnless` etc to `assertTrue`, `assertFalse`, `assertEqual` and `assertNotEqual`, all supported from Python 2.6
- I started working on Django 1.6 support, and only pushed a backward-compatible version of #36.  I still have lots of pending changes on my computer about this, but the test suite still fails so I'll stick to this bunch of changes for today.