Remove GCMNoCollapseKeyException

It turned out that collapse key is not a requirement for messages with
TTL at least since Oct, 2012.

See https://plus.google.com/u/0/104524825852741167674/posts/AEpCusdNZY9
for details.
Merge pull request #30 from imankulov/remove_collapse_key_exception

Remove GCMNoCollapseKeyException