Set desired capabilities version to return only the browser version.

According to the WebDriver spec, version should return the browser
version.  With this, only the browser version is returned, allowing for
consumers of the version value to consume it using version parsers
without having to perform additional string manipulation to parse out
text and GhostDriver version information.
Merge pull request #176 from nathandace/master

Set Capability browserVersion to return only the PhantomJS version