Don't put full file paths into docstrings (IERS)

The IERS API docs, when rendered in Sphinx look like:

> classmethod read(file='/home/mdboom/Work/builds/astropy/build/lib.linux-x86_64-2.7/astropy/time/iers/data/eopc04_IAU2000.62-now.gz', readme='/home/mdboom/Work/builds/astropy/build/lib.linux-x86_64-2.7/astropy/time/iers/data/ReadMe.eopc04_IAU2000', data_start=14)

That is, they include full paths to files on the machine that built
the docs.

This PR remedies that.