Update travis tox usage more

Author: Daniel Gallagher

.travis.yml

@@ -1,15 +1,26 @@
 language: python
-python: 3.6
-env:  # These should match the tox env list.
-    - TOXENV=flake8
-    - TOXENV=py27
-    - TOXENV=py33
-    - TOXENV=py34
-    - TOXENV=py35
-    - TOXENV=py36
-    - TOXENV=pypy
-    - TOXENV=pre-commit
-
+matrix:
+  include:
+    -  env: TOXENV=py27
+    -  env: TOXENV=py33
+       python: 3.3
+    -  env: TOXENV=py34
+       python: 3.4
+    -  env: TOXENV=py35
+       python: 3.5
+    -  env: TOXENV=py36
+       python: 3.6
+    -  env: TOXENV=pypy
+       python: pypy-5.7.1
+    -  env: TOXENV=pre-commit
+       python: 3.6
+install: pip install coveralls tox
+script: tox
+after_success: coveralls
+cache:
+  directories:
+    - $HOME/.cache/pip
+    - $HOME/.cache/pre-commit
 deploy:
   provider: pypi
   user: syrusakbary