* Plugins can now explicitly declare supported and incompatible Moodle
  versions in version.php
  - $plugin->supported[37,39];
    supported takes an array of ascending numbers, that correspond to a
    range of branch numbers of supported versions, inclusive. Moodle
    versions that are outside of this range will produce a message
    notifying at install time, but will allow for installation.

  - $plugin->incompatible = 36;
    incompatible takes a single int corresponding to the first incompatible
    branch. Any Moodle versions including and below this will be prevented
    from installing the plugin, and a message will be given when
    attempting installation.