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"
Merge branch 'master' into travisci
Fix SDL2 build regression caused by undefined Key usage

3e075586ead63816d93c5df5871a4a11bbd85edd ("To prevent undesirable release of
analogue joystick while using touchpad.") used SDLK_LSUPER from SDL1.2 which is
not used anymore in mupen64plus-input-sdl. Instead SDL2 keys must be used and
only the SDL1.2 compatibility wrapper can reference SDLK_*