Add travis support + code cleanup / simplification

Please add this package to the list of packages tested by travis.
I'm using hydro for the travis test.
I aggree that this commit will not correct the already existing issue on the analysis of the list actuated joints, that was mentioned in https://github.com/laas/jrl_dynamics_urdf/pull/13.
The only difference is that this simplifies the commit f1388aeb1d84c75c87d98f89a8c67e78cfdd7a79 by removing an unnecessary function call.
If this commit is more confusing than helping, I can remove it from the PR.