travis-ci: enable python3.5

https://github.com/fphammerle/free-disk/pull/1/commits/50a3806ab8e6a07d30935ba5bd74c9b95208a25b

.travis.yml

@@ -1,6 +1,7 @@
 language: python
 
 python:
+- 3.5
 - 3.6
 - 3.6-dev
 - 3.7