Author: Paul Holden
MDL-66917 admin: validate uploaded plugin version.
Prevent overwriting current plugins with older versions of themselves.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=59f941f6dbdf622ad81de1ad379ff82a80f8d461
http://github.com/moodle/moodle/commit/59f941f6dbdf622ad81de1ad379ff82a80f8d461
+ 3.9 en [validationmsg_pluginversiontoolow,core_plugin]