Make the project pass Travis builds & change Travis config a bit

I've made some modifications with which the project passes on Travis, see [here](https://travis-ci.org/Underyx/flask-redis/builds/19228385).

I also changed build targets a bit:

 - Python 3 is now tested against, as the current code is compatible.
   - Flask >=0.10 is required, though, 0.9 doesn't seem to work under Python 3.
 - Redis 2.4.13 is not supported, as it doesn't have the `from_url`  method.