Change wcs tests to use NumpyRNGContext

Recently I've noticed a travis has occasionally been failing due to some sort of convergence problem in the WCS tests. I speculate that this *may* be due to randomness in some part of the WCS code. So in an attempt to rectify this, I've wrapped parts of the code that might be at fault in the `NumpyRNGContext`.  It's still possible that something non-deterministic is happening that does *not* use the numpy RNG, in which case this won't help.  But it's worth a shot at least.

Unfortunately, I think the only way to test this is to merge it and just keep an eye on tests for a while, as I've seen this error at roughly a 1/50 job sort of rate...

@mdboom or @nden - do you think this is plausible?