Changes to make build independent of version of astropy

Fixed already, uses astropy.__version__

From: Erik Bray <notifications@github.com<mailto:notifications@github.com>>
Reply-To: spacetelescope/sphere <reply@reply.github.com<mailto:reply@reply.github.com>>
Date: Thursday, July 24, 2014 at 12:54 PM
To: spacetelescope/sphere <sphere@noreply.github.com<mailto:sphere@noreply.github.com>>
Cc: Bernard Simon <bsimon@stsci.edu<mailto:bsimon@stsci.edu>>
Subject: Re: [sphere] Changes to make build independent of version of astropy (#36)


_last_generated_version is very much an internal detail that shouldn't be used.