Drop tests for Python 2.6 and add Django 1.7

We will not support Python 2.6 from now on.

tox.ini

@@ -1,10 +1,6 @@
 [tox]
 envlist =
-    py26-django16,
-    py26-django15,
-    py26-django14,
-    py26-django13,
-    py26-django12,
+    py27-django17,
     py27-django16,
     py27-django15,
     py27-django14,
@@ -15,38 +11,10 @@ envlist =
 changedir = example
 setenv = PYTHONPATH = {toxinidir}/example
 
-[testenv:py26-django16]
+[testenv:py27-django17]
 commands = django-admin.py test cumulus --settings=settings.test
 deps =
-    Django>1.5,<1.7
-    pyrax>=1.9,<1.10
-    Pillow
-
-[testenv:py26-django15]
-commands = django-admin.py test cumulus --settings=settings.test
-deps =
-    Django>1.4,<1.6
-    pyrax>=1.9,<1.10
-    Pillow
-
-[testenv:py26-django14]
-commands = django-admin.py test cumulus --settings=settings.test
-deps =
-    Django>1.3,<1.5
-    pyrax>=1.9,<1.10
-    Pillow
-
-[testenv:py26-django13]
-commands = django-admin.py test cumulus --settings=settings.test
-deps =
-    Django>1.2,<1.4
-    pyrax>=1.9,<1.10
-    Pillow
-
-[testenv:py26-django12]
-commands = django-admin.py test cumulus --settings=settings.legacy
-deps =
-    Django>1.1,<1.3
+    Django>1.6,<1.8
     pyrax>=1.9,<1.10
     Pillow