Update .travis.yml

Use the magic `TOXENV=py` to automatically infer the python version from which tox is being run. Makes much sleeker travis config. Also, don't need to include the python version in the include because it knows to pick the first one in the list

.travis.yml

@@ -4,27 +4,20 @@ cache: pip
 language: python
 
 python:
- - "2.7"
- - "3.3"
- - "3.4"
- - "3.5"
- - "3.6"
+ - 3.6
+ - 3.5
+ - 3.4
+ - 3.3
+ - 2.7
+
 env:
-  - TOXENV=manifest
- - TOXENV=docs
+ matrix:
+ - TOXENV=py
 
 matrix:
  include:
- - python: "2.7"
- env: TOXENV=py27
- - python: "3.3"
- env: TOXENV=py33
- - python: "3.4"
- env: TOXENV=py34
- - python: "3.5"
- env: TOXENV=py35
- - python: "3.6"
- env: TOXENV=py36
+ - env: TOXENV=docs
+ - env: TOXENV=manifest
  allow_failures:
  - python: "3.3"
  - python: "3.4"