Fix support for running on Travis-CI

Currently Travis-CI doesn't support test-kitchen or vagrant VMs well,
and so including the kitchen tasks causes [sad errors like this][1].

This patch only includes kitchen Rake tasks if NOT running on Travis-CI.
(i.e.: TRAVIS environment variable is not set)

After this patch, we get [happy results like these][2]

[1]: https://travis-ci.org/trinitronx/mythtv-cookbook/jobs/16244171
[2]: https://travis-ci.org/trinitronx/mythtv-cookbook/builds/16244506