Move install into external shell script. We cannot use apt-get on OSX, use brew instead... Also valgrind isn't used at the moment so skip intalling it.
When fixing the indentation for the coverity chagnes in 924bcfbc99
the build matrix got turned off. This re-enables this.
Also change from using gcc/clang. What we want is to build on both Linux
and OSX. Not just run Linux with gcc+clang.
However to enable multi-os support one has to mail the travis support:
http://docs.travis-ci.com/user/multi-os/
"This feature needs to be enabled manually. If you wish to take advantage
of this feature, please send email to support@travis-ci.com, indicating
which repository you would like to have it enabled."
