Reduce number of travis tests

We don't need to test all database backends with Python 2.6.

.travis.yml

@@ -9,6 +9,16 @@ env:
   - DJANGO_VERSION=1.5.1 DATABASE=mysql
   - DJANGO_VERSION=1.5.1 DATABASE=postgresql
   - DJANGO_VERSION=1.5.1 DATABASE=sqlite
+matrix:
+  exclude:
+    - python: 2.6
+      env: DJANGO_VERSION=1.4.5 DATABASE=mysql
+    - python: 2.6
+      env: DJANGO_VERSION=1.4.5 DATABASE=postgresql
+    - python: 2.6
+      env: DJANGO_VERSION=1.5.1 DATABASE=mysql
+    - python: 2.6
+      env: DJANGO_VERSION=1.5.1 DATABASE=postgresql
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install git libcairo2-dev libgtk2.0-dev libglib2.0-dev libtool libpango1.0-dev libatk1.0-dev libffi-dev libpq-dev libmysqlclient-dev