make clean does not fail anymore when no doc.

A bit silly, but working ok : create a script then execute and remove
it, allow me to test existence before removing.
I will therefore add it to travis testing !
Travis now tests 'make doc' and 'make clean'.
Merge branch 'master' of github.com:GL-MPRI-2014/GL_MPRI_2014
Picture is now a link to the history of Travis

It used to be just a link to the picture somewhere on their servers.
Merge branch 'master' into interface_dev