Simplify Travis CI configuration

Travis now support Python 3.7.

Author: jdufresne

.travis.yml

@@ -2,6 +2,7 @@ language: python
 python:
   - "pypy3"
   - "pypy"
+  - "3.7"
   - "3.6"
   - "3.5"
   - "2.7"
@@ -16,18 +17,6 @@ env:
     - TOXENV=base
     - TOXENV=six19-optional
 
-matrix:
-  include:
-    - python: "3.7"
-      dist: xenial  # required for Python >= 3.7
-      env: TOXENV=optional
-    - python: "3.7"
-      dist: xenial  # required for Python >= 3.7
-      env: TOXENV=base
-    - python: "3.7"
-      dist: xenial  # required for Python >= 3.7
-      env: TOXENV=six19-optional
-
 install:
   - pip install tox codecov