removed a number of TRAVIS builds

This PR removes all builds with Astropy stable and limits testing to 2.7 and 3.3. At this stage `specutils` does not have a release, which would be tied to a specific astropy version. 

Furthermore, `specutils` develops closely with the new changes in the modelling Framework and has just switched to `astropy-helpers`. 

If there are no major objections, I will merge in 24h.