Workaround for simonmar/async#14

Pinging @simonmar. Thank you for your response in the tickets. However,
the StablePtr technique simply caused this test suite to hang
indefinitely. Instead, I'm trying to catch the BlockedIndefinitelyOnSTM
exception sent by the RTS, which seems to work reliably.

Pinging @sol. This change needs to be included in hspec as well. Without
a patch, for example, the enclosed-exceptions test suite still fails,
since hspec ends up letting the test die due to the
BlockedIndefinitelyOnSTM exception being sent to it as well. I can send
a pull request after this is reviewed/accepted into enclosed-exceptions.