GenericValue: add non-template overload for operator!=

As reported in #113, recent versions of Clang complain about
ambiguous overloads for some comparison operator instantiations,
especially if the deduced template type is a GenericValue.

Add an explicit, non-templated version for now (which is a better
match).

This only solves part of the problem, as comparisons between
  * GenericValue & GenericDocument
  * GenericValue with different SourceAllocator types
will still cause ambiguities.

include/rapidjson/document.h

@@ -685,6 +685,11 @@ public:
     */
     template <typename T> RAPIDJSON_DISABLEIF_RETURN(internal::IsPointer<T>, bool) operator==(const T& rhs) const { return *this == GenericValue(rhs); }
 
+    //! Not-equal-to operator
+    /*! \return !(*this == rhs)
+     */
+    bool operator!=(const GenericValue& rhs) const { return !(*this == rhs); }
+
     //! Not-equal-to operator with arbitrary types
     /*! \return !(*this == rhs)
      */