Remove Jenkins Build Status from README

This is quite straightforward - it just removes the testing badge for jenkins from the README (and thereby also http://github.com/astropy/astropy ), and is necessary because Shining Panda is now defunct.

At some point there was discussion of setting up our own jenkins, but I'm thinking it makes sense to remove this for now and we can always add it back in later if we think that's wise.

cc @astrofrog @mdboom @embray