ADDED: Travis CI integration

I have just hooked up Travis CI to our Github repository to enable
build testing of our code.

Henceforth, new commits will automatically trigger build testing of
the master branch across the following 10 Linux distributions:

Debian 9/10/sid
Ubuntu 14.04/16.04/17.10
Fedora 26/27
CentOS 6/7

The list of distros is updated as and when new releases are made.
These 10 distros make up the majority of our Linux install base
between them.

Successful builds are marked in the commit log with a green tick -
failed builds with a red cross:

https://github.com/XMLTV/xmltv/commits/master

Cheers,
Nick

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot