Update Django versions in .travis.yml
Split dev requirements in build and dev requirements

This will allow to refer to build requirements in .travis.yml and use
make build on it.
Use make build in .travis.yml

This should fix the assets directory absence issue, and fix bug 2751.
Remove --clear from collectstatic in Makefile since Django 1.3.x is still supported
Stop runserver in .travis.yml after 100 seconds

This should allow Travis builds to pass if no errors arise.