Fix parsing for HockeyAppVersion.ShortVersion

The ShortVersion in release email subjects has been missing.
According to http://support.hockeyapp.net/kb/services-webhooks-desktop-apps/how-to-use-hockeyapp-webhooks
the name of the json field is "shortversion", not "short_version".

This is untested.
Merge pull request #1 from sandyarmstrong/master

Fix parsing for HockeyAppVersion.ShortVersion