Update CI config to use Python 3.7 (#323)

Ref: speced/bikeshed@c6b80b5

Author: xfq

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: python
 python:
-- '2.7'
+- '3.7'
 install:
 - pip install pygments lxml setuptools --upgrade
 - git clone https://github.com/tabatkins/bikeshed.git