Use the generic ci-helpers script

[bsipocz]

Using the generic script will allow us to put more generic stuff in there without code repetition, e.g. the checks for the custom tags and skipping the builds early.

[coveralls]

Coverage remained the same at 100.0% when pulling 21c7872 on bsipocz:ci-helpers_use_generic_script into acaa02e on astropy:master.