Travis CI fixup

The Travis CI builds were failing.
 * The example show_tracks.py was timing out because it was waiting for some stdin which never appeared. I changed the default, when no parameters are given, to use the example long_track_test.dat
 * Python 3 was failing in many places because it used Python 2-style prints. Updated to use `from __future__ import print_function` so the same Python 3-style `print()`s work on both Py2 and Py3.
 * Python 3 is still failing because it doesn't like the import in \__init\__.py, but I've allowed Python 3 builds to fail for the time being (`allow_failures` in .travis.yml).
 * Added a Travis CI build badge to the README so broken code can be discovered sooner.
 * `flake8` for the modified files.

Please can you enable Travis CI and Coveralls for your account? 

Both are free, you log in with your GitHub ID:
 * https://travis-ci.org/profile/
 * https://coveralls.io/repos/new

Also, pull requests will be built so you can see the status before merging in.

Thanks!