fix: set Python version to 3.5 in .travis.yml

Travis build failed on Python 3.5 because it doesn't pre-installed
in default build environment.

See travis-ci/travis-ci#5993

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+python: 3.5
 
 install:
   - pip install coveralls tox