gemspec: leave open versions for some dependencies

Change-Id: Ibe5874a35c3472a0cb334579a5e21dc8cbb2c245
Reviewed-on: http://review.couchbase.org/31713
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>