JCLOUDS-75: Support '-incubating' release versions

See https://issues.apache.org/jira/browse/JCLOUDS-75 and http://mail-archives.apache.org/mod_mbox/incubator-jclouds-dev/201305.mbox/%3C20130520224345.tfjcrq1jkc08cg8g%40webmail.qrmedia.com%3E