- Use Python 3.5 instead of 2.7 in TravisCI.

- Cache pip and pre-commit in TravisCI.

.travis.yml

 ---
 language: python
-python: "2.7"
+python: "3.5"
 dist: trusty
 sudo: false
 
+cache:
+  - pip
+  - directories:
+      - $HOME/.pre-commit
+
 install:
   - pip install pre_commit