ERFA not scalar-proof

While trying to use the full multi-D flexibility of the new `erfa` routines in `Time`, it turns out not all parts can handle scalars:
```
In [21]: from astropy import erfa

In [22]: erfa.utctai(2400000.,0.)
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-22-27c7567c1ad0> in <module>()
----> 1 erfa.utctai(2400000.,0.)

/data/mhvk/venv/astropy-dev/lib/python3.4/site-packages/astropy-1.0.dev10476-py3.4-linux-x86_64.egg/astropy/erfa/_erfa.cpython-34m.so in astropy.erfa._erfa.utctai (astropy/erfa/erfa.c:184367)()

/data/mhvk/venv/astropy-dev/lib/python3.4/site-packages/astropy-1.0.dev10476-py3.4-linux-x86_64.egg/astropy/erfa/_erfa.cpython-34m.so in astropy.erfa._erfa.check_errwarn (astropy/erfa/erfa.c:5406)()

IndexError: 0-d arrays can't be indexed
```
Here, the problem is actually that this value of `jd` causes a warning; it works fine for a julian date that is within the expected range:
```
In [23]: erfa.utctai(2450000.,0.)
Out[23]: (array(2450000.0), array(0.0003356481481481266))
```