Setup Continuous Integration on Travis CI

Travis CI works coupled with github. Just registering public projects allow one to run continuous integration on Linux and Mac Boxes. 

Using this pull request, registering dealii on Travis CI, will make all pull requests tested on Linux (and Mac) on a public server. See 

https://travis-ci.org/luca-heltai/dealii/builds

for an example. 
Might be worthwhile replacing the '-j4' by something like '-j`cat /proc/cpuinfo | grep CPU | wc -l`' to really use all CPUs available.