Add travis-ci support

I had to add the shell used on Makefile. `pushd` and `popd` are not defined for `sh`.

This way we may know when the tests break. Also it's clear that the tests are broken.

You would just need to log on Travis-CI and enable the repository to be built and tested: http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in

Is it something desirable?