For awhile now this has been a requirement but not for all 2.x plugins: see http://docs.moodle.org/dev/Plugin_validation#Exceptions
so atm Themes and course formats don't require it in 2.x
should we require version.php for all plugins (only in 2.4?) This will aid in
MDL-35238 where we're trying to automate installation of plugins from plugins DB.
ps: editing a pluginversion(.php) allows version number editing apparently due to this flexibility requirement.