Add Travis CI configuration file for automatic build tests

Github provides a Travis CI integration which makes it easy to have a quick
check of a patch before a pull request is accepted. It is not enough to prove
the correctness of a contribution but at least helps to avoid some of the worst
build problems.

This feature *must* be enabled by the mupen64plus administrator *before this
commit is merged*:

- https://travis-ci.org/profile/mupen64plus
- Login via your Github account
- Make sure you are on "https://travis-ci.org/profile/mupen64plus"
- Switch repository to "On"