Fix detection of Elasticsearch version for Beta and RC releases

The Elasticsearch check assumes that the version is returned in semver x.y.z format, but Elasticsearch has a nasty habit of having x.y.z.STRING to denote beta releases. The side effect is that if you are using a beta or RC version, the version check will assume you are running an old version, and will 404 on various health checks (as the APIs have changed).

![image](https://cloud.githubusercontent.com/assets/3114436/4731439/09a8091a-59aa-11e4-88e3-bd5eb58212c1.png)