Unit latex representation shows powers as floats

Not a big issue, but it would be nice if powers weren't represented as floats:

```python
In [1]: from astropy import units as u, constants

In [2]: u.Unit('erg / (s Angstrom cm^2)')
Out[2]: Unit("erg / (Angstrom cm2 s)")

In [3]: u.Unit('erg / (s Angstrom cm^2)')._repr_latex_()
Out[3]: u'$\\mathrm{\\frac{erg}{\\AA\\,s\\,cm^{2.0}}}$'

In [5]: u.Unit('cm^2')._repr_latex_()
Out[5]: u'$\\mathrm{cm^{2.0}}$'

```