Added Python 3.7 to Travis/Tox.

Signed-off-by: Brett Randall <javabrett@gmail.com>

.travis.yml

@@ -8,6 +8,10 @@ matrix:
           env: TOXENV=py27
         - python: 3.6
           env: TOXENV=py36
+        - python: 3.7
+          env: TOXENV=py37
+          dist: xenial
+          sudo: true
 
 install:
     - travis_retry pip install tox

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py36
+envlist = py27,py36,py37
 
 [testenv]
 commands=python test_httpbin.py