Revert ".travis.yml: Try another way"

This reverts commit fd52e5c842.

.travis.yml

@@ -2,26 +2,15 @@ sudo: false
 
 language: python
 
-matrix:
-    include:
-        - python: 2.6
-          env: TOXENV=py26
-        - 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
-        - python: pypy
-          env: TOXENV=pypy
-        - python: 2.7
-          env: TOXENV=flake8
-        - python: 2.7
-          env: TOXENV=coverage
+env:
+    - TOXENV=py27
+    - TOXENV=py33
+    - TOXENV=py34
+    - TOXENV=py35
+    - TOXENV=py36
+    - TOXENV=pypy
+    - TOXENV=flake8
+    - TOXENV=coverage
 
 install:
     - travis_retry pip install tox