Remove DeprecationWarning superclass for AstropyDeprecationWarning

we do this because in py2.7, DeprecationWarning and subclasses are hidden by default, but we want astropy's deprecations to get shown by default
Merge pull request #1871 from eteq/astropy-deprecation-show

Make `AstropyDeprecationWarning`s appear by default in py2.7