Adapt Gammapy to recent Astropy API changes

This PR updates Gammapy to the following recent API changes in Astropy that lead to unit failures:
* The parameter `width` was renamed to `stddev` in the [astropy.convolution.Gaussian2DKernel](http://docs.astropy.org/en/latest/api/astropy.convolution.Gaussian2DKernel.html).
* The `_model_to_fit_params` method was moved from the model to the fitter class.

It also adds builds with the latest development version of Astropy to travis-ci.
(Previously Gammapy was only tested with the latest stable version of Astropy.)