Drop one test case

[mwcraig]

Drop test case astropy1.0.x (LTS) and python 3.6, see #612

[bsipocz]

Well, you know your user base, but given that one is a very old astropy branch (even though it was an LTS), I wouldn't worry about supporting it in the new ccdproc releases. It just sounds very strange that anyone would stick to an old astropy but would like to use the latest and greatest ccdproc.