Protect library from connection_pool for 1.8.7+ rubies

It work well without ConnectionPool there, because ruby 1.8 uses green
threads and doesn't need to be synchronized

Change-Id: I5803cf6438eeaf7b3ac5c484debedf335b7d48f3
Reviewed-on: http://review.couchbase.org/26883
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>