Travis CI: Add Python 3.6 and 3.7 to the testing (#549)

diff --git a/.travis.yml b/.travis.yml
index 9c6e2e2..67ce5ca 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,11 @@
   - "2.7"
   - "3.4"
   - "3.5"
+  - "3.6"
+matrix:
+  include:
+    - python: "3.7"
+      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
 install:
   python setup.py install
 script: