Setting up stages for travis

[bsipocz]

E.g. having stages would be rather useful, there is no need to run the whole matrix and populate the queue with failing jobs when there those failures would be already present in a small number of initial jobs (e.g one test job and the codestyle one).

[nmearl]

Is there an example repo that implements the travis stages?

[bsipocz]

Sure. the package-template does it, as well as e.g. astropy/photutil/astroquery etc.

I'm happy to open a quick PR if you find that helpful.