Explicitly state msgpack doesn't modify char* buffers from RSTRING_PTR

From what I could investigate, msgpack doesn't modify char* buffers
obtained from RSTRING_PTR. This means that on Rubinius we don't have to
copy back and forth the buffer to make sure it's also updated on the
Ruby side.

This copying of buffers is a similar problem as the RARRAY_PTR problem,
because it is not safe to expose GC'ed memory on Rubinius to extensions
since it can move due to Rubinius having a moving GC.

ruby/pack.c

@@ -15,6 +15,7 @@
  *    See the License for the specific language governing permissions and
  *    limitations under the License.
  */
+#define RSTRING_NOT_MODIFIED
 #include "ruby.h"
 #include "compat.h"