Make "install-dependencies.sh" reusable

* move a "sudo" command to travis installation script
* add some "git pull" in the case the repo couldn't be cloned as it exists already

This makes "install-dependencies.sh" reusable by other building
processes , e.g. when deploying to a server.
Merge pull request #631 from lobid/dependency-installation

Make "install-dependencies.sh" reusable