ci: Travis: Python 3.5.1 via Trusty

Python 3.5.0 caused flaky failures before
(pytest-dev#5795), hopefully this is
more stable.

This is pulled out of pytest-dev#6435,
which adds code specific for Python < 3.5.2, and therefore it makes
sense to test it.

.travis.yml

@@ -53,7 +53,8 @@ jobs:
       python: 'pypy3'
 
     - env: TOXENV=py35-xdist
-      python: '3.5'
+      python: '3.5.1'
+      dist: trusty
 
     # Specialized factors for py37.
     - env: TOXENV=py37-pluggymaster-xdist