Support ghc-7.2.1 by enabling FlexibleInstances

In ghc-7.2.1 the TypeSynonymInstances extension now correctly requires that instances are valid once the type synonym is expanded. This means that when Row is expanded we need to enable the FlexibleInstances extension.

src/Data/CSV/Enumerator.hs

@@ -1,6 +1,6 @@
 {-# LANGUAGE OverloadedStrings, BangPatterns #-}
 {-# LANGUAGE PackageImports #-}
-{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
 
 module Data.CSV.Enumerator 
   (