Update list of Python test envs on Travis

Author: ldx

.travis.yml

@@ -1,8 +1,11 @@
+dist: xenial
 language: python
 python:
-  - "2.6"
   - "2.7"
   - "3.4"
+  - "3.5"
+  - "3.6"
+  - "3.7"
 install:
   - python setup.py build
   - python setup.py install