Turn our travis config from a php project to a C one

Switched from a php (5.4) project to a C project
This means that now we are required to install some dependencies for ourselfs (http://docs.travis-ci.com/user/languages/c/ shows what is available for C projects by default), but this also makes our build a bit more explicit and more straightforward to debug.
As a side effect, this also makes the build a bit faster, as it won't install a php version which we won't use
I've added a separate shell script for installing the deps, and had to change our run-tests.php as it was using the php-specific travis environment variable instead of the generic one.
This change also opens up the possibility to start using the travis compilers as a build axis(gcc & clang).
I'm planning to merge this PHP-5.5 and upwards (we don't don travis builds below 5.5), just wanted to make sure that nobody objects it.