Migrate Travis to use docker to test on recent linux distributions

[traversaro]

At the moment, the robotology-superbuild is being tested on Travis on a environment that is quite different from the one actually used by most users. A solution for this would be to move Linux builds to a docker based environment, as done for example in the repo of the DART simulator https://github.com/dartsim/dart/blob/master/.travis.yml or in the wb-toolbox robotology/wb-toolbox#136 .

Given that the robotology-superbuild is supposed to build all its own non-system dependencies, I think it would make sense to just use some of the bare docker images, or perhaps refactor the images in https://github.com/robotology-playground/docker_images to just have an image with the system dependencies installed.

Given that we already have 4 builds (and I do not think it is wise to increase this number) I think we should test the superbuild on xenial, bionic and debian stable and testing.

A nice byproduct of this approach would be that we would refactor some of the build steps in bash scripts that could be reused also by the internal based Jenkins system @mbrunettini .

[traversaro]

cc @claudiofantacci