Travis CI: Add Python 3.7 to the testing

Also, [Travis are now recommending removing the __sudo__ tag](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration).

.travis.yml

@@ -1,7 +1,5 @@
 language: python
 
-sudo: false
-
 python:
   - '3.5'
   - '2.7'
@@ -9,6 +7,11 @@ python:
   - '3.4'
   - '3.6'
 
+matrix:
+  include:
+    - python: '3.7'
+      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+
 install:
 - pip install -e .
 - pip install -r requirements-dev.txt