[#33838] Generate version of atom feed from JVersion

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33838&start=0

## Testing Instructions
Check an atom feed of a component. Before patch the < generator > tag will show the version as 1.6. After patch it will show 3.3 (and pulls the version number from JVersion so will not need updating in the future)
One small detail just for the sake of 100% security habits, even if not needed for most versions: it's good practice and helps security-reviews to add around `$versionDetails` (same as done at all other relevant places, e.g. https://github.com/wilsonge/joomla-cms/blob/patch-6/libraries/joomla/document/feed/renderer/atom.php#L84 ):
`htmlspecialchars($versionDetails, ENT_COMPAT, 'UTF-8')`

So that that line would read:
`$feed .= "	<generator uri=\"http://joomla.org\"" . htmlspecialchars($versionDetails, ENT_COMPAT, 'UTF-8')  . ">" . $data->getGenerator() . "</generator>\n";`

Technically not needed as long as we follow semver, but for peace of mind and for good example, and in case we have a special version number generated, or someone has access to change a version number it can avoid a highly improbable xss.