Fixes toString forward reference issue:

- as raised in https://github.com/philsquared/Catch/pull/195

include/internal/catch_tostring.hpp

@@ -80,6 +80,9 @@ namespace Detail {
 
 } // end namespace Detail
 
+template<typename T>
+std::string toString( T const& value );
+
 template<typename T>
 struct StringMaker :
     Detail::StringMakerBase<Detail::IsStreamInsertable<T>::value> {};