Fix for rcov crash

After migrating to the new MacBook Pro, rcov started giving me grief. My unit tests would run under rcov fine, but my functionals would blow up with this lovely error:

(eval):1: [BUG] rb_gc_mark(): unknown data type 0x24(0x1ba523c) non object

Oh. Of course, right? Well, as always I consulted The Oracle, and she didn't fail me. There were a number of pages of people having this same problem, but this one had a solution that worked for me. Many thanks to Sylvestre Mergulhão!