Something wrong with ɛ0 and c0^3

Hi, guys! Thanks for a great package!

I've just installed it and suddenly faced what I believe is a bug.

Results of the following two snippets are different despite they are expected to be equivalent (the first result is correct, btw):

```julia
Unitful.c0*1u"c^2"*Unitful.ɛ0
> 2.3856725796184712e14 m^2 H^-1 s^-1

uconvert(u"m^2*H^-1*s^-1", Unitful.c0^3*Unitful.ɛ0)
> 4.864298731871295e7 m^2 H^-1 s^-1
```

It seems there is some issue with a conversion of c^-2 dimension.

Start by reproducing the two Julia snippets in Unitful.jl and compare the conversion paths for c^-2 dimensions. Trace the unit-conversion logic involved in c0, ɛ0, exponentiation, and uconvert; done means both expressions produce equivalent converted results.

Written by the indexing model from the issue text.