Travis: Django 1.7rc2; explicit build matrix

Switch to explicitly declaring build matrix, with the versions
of Python supported by each version of Django we handle.
(Exclusion list was becoming too complicated.)

.travis.yml

@@ -1,43 +1,43 @@
 language: python
-python:
-  - "2.6"
-  - "2.7"
-  - "3.2"
-  - "3.3"
-  - "3.4"
-  - "pypy"
-env:
-  - DJANGO=django==1.3
-  - DJANGO=django==1.4
-  - DJANGO=django==1.5
-  - DJANGO=django==1.6
-  - DJANGO=https://www.djangoproject.com/download/1.7c1/tarball/
 matrix:
-  exclude:
-    # Django < 1.5 not supported on python 3
-    - python: "3.2"
-      env: DJANGO=django==1.3
-    - python: "3.2"
-      env: DJANGO=django==1.4
-    - python: "3.3"
-      env: DJANGO=django==1.3
-    - python: "3.3"
-      env: DJANGO=django==1.4
-    - python: "3.4"
-      env: DJANGO=django==1.3
-    - python: "3.4"
-      env: DJANGO=django==1.4
-    - python: "pypy"
-      env: DJANGO=django==1.3
-    - python: "pypy"
-      env: DJANGO=django==1.4
-    # Django >= 1.5 strongly recommends python 2.7 or later (so skip 2.6)
+  include:
+    # Django 1.3: Python 2.6--2.7
     - python: "2.6"
+      env: DJANGO=django==1.3
+    - python: "2.7"
+      env: DJANGO=django==1.3
+    # Django 1.4: Python 2.6--2.7
+    - python: "2.6"
+      env: DJANGO=django==1.4
+    - python: "2.7"
+      env: DJANGO=django==1.4
+    # Django 1.5: Python 2.7, pypy
+    #   (As of Django 1.5, Python 2.6 no longer "highly recommended",
+    #   and Python 3.2+ support was only "experimental", so skip those.)
+    - python: "2.7"
       env: DJANGO=django==1.5
-    - python: "2.6"
+    - python: "pypy"
+      env: DJANGO=django==1.5
+    # Django 1.6: Python 2.7--3.3, pypy
+    - python: "2.7"
       env: DJANGO=django==1.6
-    - python: "2.6"
-      env: DJANGO=https://www.djangoproject.com/download/1.7c1/tarball/
+    - python: "3.2"
+      env: DJANGO=django==1.6
+    - python: "3.3"
+      env: DJANGO=django==1.6
+    - python: "pypy"
+      env: DJANGO=django==1.6
+    # Django 1.7: Python 2.7--3.4, pypy
+    - python: "2.7"
+      env: DJANGO=django==https://www.djangoproject.com/download/1.7c2/tarball/
+    - python: "3.2"
+      env: DJANGO=django==https://www.djangoproject.com/download/1.7c2/tarball/
+    - python: "3.3"
+      env: DJANGO=django==https://www.djangoproject.com/download/1.7c2/tarball/
+    - python: "3.4"
+      env: DJANGO=django==https://www.djangoproject.com/download/1.7c2/tarball/
+    - python: "pypy"
+      env: DJANGO=django==https://www.djangoproject.com/download/1.7c2/tarball/
 install:
   - pip install -q $DJANGO
   - pip install .