Remove unsupported Django versions from build matrix

Author: federicobond

.travis.yml

@@ -6,9 +6,6 @@ python:
   - "3.4"
   - "3.5"
 env:
-  - DJANGO='Django>=1.4,<1.5' SKIP='3.3 3.4 3.5'
-  - DJANGO='Django>=1.5,<1.6' SKIP='3.3 3.4 3.5'
-  - DJANGO='Django>=1.6,<1.7' SKIP='3.4 3.5'
   - DJANGO='Django>=1.7,<1.8' SKIP='2.6 3.5'
   - DJANGO='Django>=1.8,<1.9' SKIP='2.6'
   - DJANGO='Django>=1.9,<1.10' SKIP='2.6 3.3'