Travis CI: add Python 3.7 build target (#416)

Also set up a build matrix.

Closes #416.

Change-Id: I19c32c869fcf56650ffc72a42c8605a44c23c94b

Author: thiell

.travis.yml

@@ -1,9 +1,17 @@
+os: linux
 language: python
-python:
-  - '2.7'
-  - '3.4'
-  - '3.5'
-  - '3.6'
+matrix:
+  include:
+  - dist: bionic
+    python: 2.7
+  - dist: xenial
+    python: 3.4
+  - dist: xenial
+    python: 3.5
+  - dist: bionic
+    python: 3.6
+  - dist: bionic
+    python: 3.7
 install:
   - pip install coverage
   - pip install .