Add Travis CI configuration file for automatic build tests

[ghost]

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"