Use trusty.

Rémy HUBSCHER · Rémy HUBSCHER · commit 5bb2f1a12ad4 · 2018-02-21T06:41:32.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
+dist: trusty
 language: python
-python:
-  - "3.5"
+python: 3.5
 script: make test
 after_success: coveralls
 matrix:
