Add .travis.yml for Travis CI (for Github user...)

4 jobs
* autotools / gcc
* autotools / clang
* cmake / gcc
* cmake / clang

Change-Id: Ifc5b079415232b77680c87b2dee441566e08c835
Reviewed-on: https://code.wireshark.org/review/3944
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>