Update astropy-helpers to v1.1.2

[astrofrog]

This is an automated update of the astropy-helpers submodule to v1.1.2. This includes both the update of the astropy-helpers sub-module, and the ah_bootstrap.py file, if needed.

This is intended to be helpful, but if you would prefer to manage these updates yourself, or if you notice any issues with this automated update, please let @astrofrog know!

[coveralls]

Coverage remained the same at 100.0% when pulling f4502ba on astrofrog:update-helpers-v1.1.2 into e28afa4 on astropy:master.

[cmccully]

Hi @astrofrog. I noticed that this caused the sphinx build to fail because of two warnings:
WARNING: while setting up extension astropy_helpers.sphinx.ext.autodoc_enhancements: directive 'autoattribute' is already registered, it will be overridden
WARNING: sphinx.ext.pngmath has been deprecated. Please use sphinx.ext.imgmath instead.

Should this be fixed upstream in astropy-helpers?

[astrofrog]

I'm going to close this because astropy-helpers 1.2 is now released, so I'm going to open a new PR to replace this one. It may be the issues you are seeing will be fixed there.