Rhino 1.7R4 compatibility

Currently Spec does not run with Rhino 1.7R4, which is the current version in at least the arch AUR. This patch fallbacks to synchronous behavior on failure. I'm entirely unfamiliar with Rhino so there might be a nicer way to do this?

More info:
https://groups.google.com/forum/#!msg/mozilla-rhino/s1R791wF270/YllAxSHCJQgJ

Tested in:
- Rhino 1.7R4
- Rhino 1.7R5