travis: Resolve issues with Python 3.7

travis: Resolve issues with Python 3.7

Bump the distro version to xenial, as this is the first version to
include Python 3.7 support. Bionic is also available but it doesn't
support Python 3.5 [1].

In addition, skip a test that was valid on Python 3.4 - 3.6 but does not
appear to be an issue for Python 3.7.

[1] https://docs.travis-ci.com/user/reference/bionic/#python-support

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 6267a5fa ("Drop support for Python 3.4, add Python 3.7")