chore: retry certain steps on TravisCI to get more stable builds

While I'm still looking into ways of getting more stable Sauce tests
there is one simple trick that we can do to increase our chances 
of getting stable builds, see:
http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/

Of course this is more of a work-around than a proper solution
but I'm afraid that we might need tricks like this one given the 
number of different network-connected machines involved in the 
build process. This seems to be a conclusion from the Travis-CI guys
as well...