Refactored .travis.yml file, changed install.sh URI

Clone Gist repository instead of downloading RAW files, using pushd/popd
instead of cd and we don't need chmod when we just execute the script
using bash.

signed-off-by: Daniel Miranda <danielkza2@gmail.com>
Update install.sh script

We are using tags on the install.sh's Gist, so we know we are using the
correctly version instead the current master.

signed-off-by: Daniel Miranda <danielkza2@gmail.com>
Remove libzmq3 package installed by default on Travis

This package is bugged, it conflicts with libzmq that is a dependency
off libzeromq-perl (that itself is a dependency of Analizo).

signed-off-by: Daniel Miranda <danielkza2@gmail.com>
Enable Travis cache

This should speed the builds, see
http://docs.travis-ci.com/user/caching/ for details.