Switch by default to SDL2 for Mupen64Plus 2.1

The current version of SDL is 2.0.3. All new releases of distributions contain
this library already. Current Linux distributions and OpenBSD even backporting
patches from Mupen64Plus 2.1 repo to use SDL 2. Android and other mobile
platform don't have support for legacy SDL 1.2.

People like Anthony J. Bentley and Riley Labrecque already requested
a new release of Mupen64Plus 2.1 with SDL2
SDL 1.2 is not deprecated for us, and we shouldnt frighten the user with 6 lines of exclamation points
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 pull request #2 from fayvel/travisci

Add Travis CI configuration file for automatic build tests
Merge from upstream