.travis.yml: Switch to only matrix.include

to cover all the different Python versions

Author: msabramo

.travis.yml

@@ -1,15 +1,18 @@
 language: python
-
-env:
-    - TOXENV=py27
-    - TOXENV=py33
-    - TOXENV=py34
-    - TOXENV=py35
-    - TOXENV=pypy
-    - TOXENV=flake8
-    - TOXENV=coverage
 matrix:
     include:
+        - python: 2.7
+          sudo: true
+          env: TOXENV=py27
+        - python: 3.4
+          sudo: true
+          env: TOXENV=py34
+        - python: 3.5
+          sudo: true
+          env: TOXENV=py35
+        - python: 3.6
+          sudo: true
+          env: TOXENV=py36
         - python: 3.7
           dist: xenial
           sudo: true