Fix CI problems

[MSeifert04]

It seems like we shouldn't use python: but instead use the PYTHON_VERSION variable.

I also incremented the NumPy versions (no particular reason except that we were missing NumPy 1.15 and I think that would be more appropriate than NumPy 1.13).

Closes #706

Ref: #707

[MSeifert04]

There's one problem that needs fixing upstream (astropy/astropy#9664).

[mwcraig]

This looks good except for the one we can't fix yet...

[bsipocz]

I think we should just add pytest-astropy-header as a hard dependency rather than doing this nested try except (it should ideally be picked up from pytest-astropy, but clearly something is not working as it should).

[MSeifert04]

I think we should just add pytest-astropy-header as a hard dependency

Yeah, I also thought about this. But the test-log shows that it's installed (corresponding log line from your PR) but not picked up - which makes it probably a lot harder to solve than just adding a hard-dependency.

So I would prefer the nested try-catch for now, it's only astropy-2.x after all and we may even drop support for it once the new astropy LTS is out.

[bsipocz]

I would rather not do a double try/except, but don't have no strong preference to clean it up right now rather than when you remove python2 support anyway.

[MSeifert04]

Should this stay open until the upstream is fixed?

Otherwise I would temporarily set the astropy-dev test to allow-fail and put this in.

[bsipocz]

I would vote for merging this (CI on master is already broken, this at least fixes some issues).

[MSeifert04]

@mwcraig I'm going to merge this tomorrow if you won't object. The astropy-dev job is then (temporarily) allowed to fail.