readertest: Suppress "dangling-else" warning on GCC 7 and later

GCC 6.x doesn't yet support this warning flag, as reported by
@ragnar-ouchterlony.

test/unittest/readertest.cpp

@@ -28,7 +28,7 @@ RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(effc++)
 RAPIDJSON_DIAG_OFF(float-equal)
 RAPIDJSON_DIAG_OFF(missing-noreturn)
-#if __GNUC__ >= 6
+#if __GNUC__ >= 7
 RAPIDJSON_DIAG_OFF(dangling-else)
 #endif
 #endif // __GNUC__