instance (m ~ IO) => Example (PropertyT m a) is overly restrictive

[parsonsmatt]

Apparently we've been running hspec-hedgehog-0.0.1.2 at work until recently, because we just saw an error where we'd defined instance Example (PropertyT SqlSpec a) locally. We use it for tests like:

checkThing gen = do
  a <- forAll gen
  q <- lift $ select (f a)
  a === t q

This is related to #17 and #18 because I think there may be other/better ways of dealing with type inference.