Wrong sum of fractions in the powers of units

Hi, 
I'm having trouble with products of units having fractional powers.

E.g.
```
In [4]: u.cm ** Fraction(1,2) * u.cm ** Fraction(2, 3)
Out[4]: Unit("cm")
```
instead of `cm(7/6)`.

It is very weird, in most fractional units operations it works pretty well.
I'm on a Mac, Mavericks, with Astropy 0.4.2, python 2.7.8, numpy 1.9.0

Thanks in advance,
Matteo