add python 3.7 builds, remove 3.4

Author: miguelgrinberg

.travis.yml

@@ -1,9 +1,10 @@
+dist: xenial
 language: python
 python:
   - 2.7
-  - 3.4
   - 3.5
   - 3.6
+  - 3.7
   - pypy
   - pypy3
 install: python setup.py install