Travis Integration: Add travis file, and other minor fixes to ensure build works

The new .travis.yml file contains all the instructions travis-ci needs to
build and test stb-tester.  I've dry-run it on my public fork of stb-tester
(as can be seen here: https://travis-ci.org/stestagg/stb-tester/builds/6815758).

In order to accomodate some differences between Fedora and Ubuntu, I had
to tweak the pyflakes settings.  Some of the (more pedantic) exceptions have been moved
into extra/pylint.sh rather than being repeatedly declared inline.

I also slightly refactored the two context manager classes to use
collections.contextmanager, which is a more common pattern, and also removes some
pylint complaints. The VirtualRemote listener wait time was also increased slightly
to prevent some intermittent failures I was seeing with the travis VMs.