[.travis.yml] add Py3.7 testing, eliminate Py2.7, Py3.4 testing

Author: chrissimpkins

.travis.yml

@@ -4,14 +4,13 @@ sudo: false
 language: python
 matrix:
   include:
-    - python: 2.7
-      env: TOX_ENV=py27
-    - python: 2.7
-      env: TOX_ENV=py34
     - python: 3.5
       env: TOX_ENV=py35
     - python: 3.6
       env: TOX_ENV=py36
+    - python: 3.7
+      env: TOX_ENV=py37
+      dist: xenial
 
 script: tox -e $TOX_ENV