Fix for GHC 8.6

See https://github.com/haskell-infra/hackage-trustees/issues/356

Control/Monad/Trans/Class.hs

@@ -5,7 +5,7 @@
 #if __GLASGOW_HASKELL__ >= 710 && __GLASGOW_HASKELL__ < 802
 {-# LANGUAGE AutoDeriveTypeable #-}
 #endif
-#if __GLASGOW_HASKELL__ >= 808
+#if __GLASGOW_HASKELL__ >= 806
 {-# LANGUAGE QuantifiedConstraints #-}
 #endif
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