Fix GSL testing

As tested at the beginning of `src/RandomMatrices.jl`, `_HAVE_GSL` was always `false`; the try was failing because it was returning `_HAVE_GSL` (why?), which was not defined yet.

Also, instead of testing for `_HAVE_GSL`, `HaarSymbolic.jl` simply did a `using GSL`.  This PR makes that testing consistent with `Haar.jl`.

This should probably have just been a bug report, since it's unclear to me if you want GSL to be an actual requirement or not.  If you just want to require it, this is the wrong fix (but it's still broken).

Cheers!