RCBC-136 Build shared object for ruby 2.0

Also fixes build script when using latest rake and rake-compiler

Change-Id: I109dd07d901c5054499ff892383613bb932a2ff9
Reviewed-on: http://review.couchbase.org/28344
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>