Specify Python version for tests

Python 3.5 is only available if requested, so Python 3.5 tests were
failing. Explicit is better than implicit anyway.

.travis.yml

@@ -3,35 +3,57 @@ language: python
 env:
     global:
         - COVERAGE=1 RUNTEST_ARGS="-v --noinput"
-    matrix:
+matrix:
+    include:
         # Quality checks
-        - TOXENV=flake8
-        - TOXENV=docs
+        - env: TOXENV=flake8
+          python: "2.7"
+        - env: TOXENV=docs
+          python: "2.7"
         # Supported Python / Django versions w/ SQLite
-        - TOXENV=py27-django-110
-        - TOXENV=py35-django-110
-        - TOXENV=py27-django-19
-        - TOXENV=py35-django-19
-        - TOXENV=py27-django-18
-        - TOXENV=py34-django-18
+        - env: TOXENV=py27-django-110
+          python: "2.7"
+        - env: TOXENV=py35-django-110
+          python: "3.5"
+        - env: TOXENV=py27-django-19
+          python: "2.7"
+        - env: TOXENV=py35-django-19
+          python: "3.5"
+        - env: TOXENV=py27-django-18
+          python: "2.7"
+        - env: TOXENV=py34-django-18
+          python: "3.4"
         # Test with PostgreSQL
-        - TOXENV=py27-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-110-postgres"
-        - TOXENV=py35-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-110-postgres"
-        - TOXENV=py27-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-19-postgres"
-        - TOXENV=py35-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-19-postgres"
-        - TOXENV=py27-django-18-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-18-postgres"
+        - env: TOXENV=py27-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-110-postgres"
+          python: "2.7"
+        - env: TOXENV=py35-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-110-postgres"
+          python: "3.5"
+        - env: TOXENV=py27-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-19-postgres"
+          python: "2.7"
+        - env: TOXENV=py35-django-19-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-19-postgres"
+          python: "3.5"
+        - env: TOXENV=py27-django-18-postgres DATABASE_URL="postgres://postgres@localhost:5432/py27-django-18-postgres"
+          python: "2.7"
         # Test with MySQL
-        - TOXENV=py27-django-110-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-110-mysql"
-        - TOXENV=py35-django-110-mysql DATABASE_URL="mysql://travis@localhost:3306/py35-django-110-mysql"
-        - TOXENV=py27-django-19-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-19-mysql"
-        - TOXENV=py35-django-19-mysql DATABASE_URL="mysql://travis@localhost:3306/py35-django-19-mysql"
-        - TOXENV=py27-django-18-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-18-mysql"
+        - env: TOXENV=py27-django-110-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-110-mysql"
+          python: "2.7"
+        - env: TOXENV=py35-django-110-mysql DATABASE_URL="mysql://travis@localhost:3306/py35-django-110-mysql"
+          python: "3.5"
+        - env: TOXENV=py27-django-19-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-19-mysql"
+          python: "2.7"
+        - env: TOXENV=py35-django-19-mysql DATABASE_URL="mysql://travis@localhost:3306/py35-django-19-mysql"
+          python: "3.5"
+        - env: TOXENV=py27-django-18-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-18-mysql"
+          python: "2.7"
         # Django master is allowed to fail
-        - TOXENV=py27-django-master
-        - TOXENV=py35-django-master
-        - TOXENV=py27-django-master-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-master-mysql"
-        - TOXENV=py35-django-master-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-master-postgres"
-matrix:
+        - env: TOXENV=py27-django-master
+          python: "2.7"
+        - env: TOXENV=py35-django-master
+          python: "3.5"
+        - env: TOXENV=py27-django-master-mysql DATABASE_URL="mysql://travis@localhost:3306/py27-django-master-mysql"
+          python: "2.7"
+        - env: TOXENV=py35-django-master-postgres DATABASE_URL="postgres://postgres@localhost:5432/py35-django-master-postgres"
+          python: "3.5"
     allow_failures:
         # Master is allowed to fail
         - env: TOXENV=py27-django-master