Update astropy-helpers to v2.0.6

[bsipocz]

This is an automated update of the astropy-helpers submodule to v2.0.6. This includes both the update of the astropy-helpers sub-module, and the ah_bootstrap.py file, if needed.

A full list of changes can be found in the changelog.

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 @bsipocz or @astrofrog know!

Along with with the astropy core release v3.0, we have also started to release astropy-helpers v3.0.x versions. Similarly to the core package, these require Python 3.5+. We will open automated update PRs with astropy-helpers v3.0.x only for packages that specifically opt in for it when they start supporting Python 3.5+ only. Please let @bsipocz or @astrofrog know or add your package to the list in https://github.com/astropy/astropy-procedures/blob/master/update-affiliated/helpers_3.py

[coveralls]

Coverage remained the same at 100.0% when pulling 5180fc1 on bsipocz:update-helpers-v2.0.6 into d5f6e47 on astropy:master.