Update to 0.2.3.1

changes:
0.2.3.1
 - Remove support for compilers other than GHC
 - Make GHC >= 7.2 imports conditional
 - Data.HashMap.Array: Add import list for GHC.Exts
     GHC 7.7 now exports fromList/toList for GHC.Exts which conflict with
     names defined in this module.
 - benchmarks: rename hashmap/HashMap to hashmap/Map
 - Add benchmarks for hashmap package comparison
 - Eta-expand some worker functions
     Prior to this change, the computation of the hash
       h0 = hash k0
     would create an extra thunk in every call to e.g. 'lookup'.
 - Drop support for GHC 7.0