Merge pull request #2805 from mdboom/config/always-write

Astropy 0.4 rewriting astropy.cfg all the time, leading to race conditions
Conflicts:

	CHANGES.rst
Merge pull request #2671 from embray/issue-2671

ImportErrors possible within AstropyLogger
Conflicts:

	CHANGES.rst
Merge pull request #2825 from embray/astropy-helpers-update

Update astropy_helpers to the commit that will be tagged v0.4.1
Conflicts:

	CHANGES.rst
Merge pull request #2827 from embray/astropy-helpers-update

Update ah_bootstrap.py from soon-to-be astropy-helpers v0.4.1