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. 
Oh, I see. The results page showed that the machine had 16 cores, but this may just be what the VM reports, not how many the underlying hypervisor actually assigns.