Use Travis CI to generate the images

[diegoferigo]

Despite dockerhub offers a nice interface for handling images, their provided resources for the automatic generation are too restricted and the build times last forever.

We should switch to Travis asap.

[diegoferigo]

A new image based on the robotology-superbuild has been pushed in 74157e0.

A new Travis CI stage for generating the image has been added in 35e0789. If this pipeline works well, we can exploit stages to build also the other images. This would ensure the order of the Travis jobs, and allows building the entire pipeline yarp > yarp-gazebo > yarp-tdd.

A nice result of this approach is that the Dockerfile can have input arguments. This allows to keep just one file and generate from it many images e.g. for different gazebo versions.

Note: considering that robotology-tdd would be the image used by the CI pipeline of our projects, we can reconsider if it is still worth shipping yarp-tdd and yarp-gazebo.