drop support for django < 1.3.7

Author: michaelkuty

.travis.yml

@@ -13,7 +13,6 @@ script:
   - coverage run --branch --source=dbtemplates `which django-admin.py` test dbtemplates
   - coverage report --omit="dbtemplates/test*,dbtemplates/migrations*"
 env:
-  - DJANGO=1.3.7
   - DJANGO=1.4.5
   - DJANGO=1.5.1
   - DJANGO=1.7.8