Updating tests to run against latest Django versions (1.4.10/1.5.5/1.6)

Removed 1.2 since usage for that is very low. Updated 1.4/1.5 to the latest point release and added tests for 1.6
Merge pull request #138 from nickcatal/patch-1

Updating TravisCI to run against latest Django versions (1.3.7/1.4.10/1.5.5/1.6)