Update Travis CI config

Author: hackebrot

.travis.yml

@@ -1,32 +1,29 @@
-# Config file for automatic testing at travis-ci.org
-
-sudo: false
+dist: xenial
 language: python
 
+cache:
+  directories:
+    - $HOME/.cache/pip
+
 matrix:
     include:
       - python: 2.7
-        env: TOX_ENV=py27
+        env: TOXENV=py27
       - python: 3.4
-        env: TOX_ENV=py34
+        env: TOXENV=py34
       - python: 3.5
-        env: TOX_ENV=py35
+        env: TOXENV=py35
       - python: 3.6
-        env: TOX_ENV=py36
+        env: TOXENV=py36
+      - python: 3.7
+        env: TOXENV=py37
       - python: pypy
         env: TOX_ENV=pypy
-      - python: 3.6
-        env: TOX_ENV=flake8
+      - python: 3.7
+        env: TOXENV=flake8
 
 install:
   - pip install tox
 
 script:
-  - tox -e $TOX_ENV
-
-before_cache:
-  - rm -rf $HOME/.cache/pip/log
-
-cache:
-  directories:
-    - $HOME/.cache/pip
+  - tox