Fix gemspec warnings: honour semantic versions

Change-Id: Ie04499561d29ee9cedbc8132dcca28d3506a3759
Reviewed-on: http://review.couchbase.org/31684
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>