Use xenial distribution for Travis CI
Use xenial distribution for Travis CI

This allows moving the required version of Python from 3.4, which has
reached EOL, and which pip now deprecates, to 3.5.

Simplify the test matrix, since there is no obvious benefit to having
it be more complicated.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratis-cli,stratis-storage/stratis-cli