Unfreeze rake version

Change-Id: Ie19ffd85be82b27caa4fd149a3b13fec27f4f518
Reviewed-on: http://review.couchbase.org/24479
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
RCBC-27 EventMachine plugin

Introduce plugin which allows to use EventMachine as event loop for
couchbase client.

With Ruby 1.9 and Fibers it is possible to use blocking interface,
otherwise connection should be asynchronouse and connection.run should not
be used.

Change-Id: I6a5bd7c86f9ce5f3b7090b0c58bc411d6b6a52f7