[s1904] Add continued integration support using Travis CI

Every push to a repository will start a new build on travis which will
make sure that CMaNGOS still builds. Currently, Travis only supports gcc
and clang on linux, Windows and OS X build test is not yet available.

Thanks to @travis-ci for their awesome software and their
sponsors for providing the infrastructure!
For the current build status of this repository, go to https://travis-ci.org/cmangos/mangos-wotlk/

(based on commit [12369] - 7349c20)