Improved Server Version Check

This ensures that server administrators are only using this on CraftBukkit
1.6.4-R2.0 or 1.6.4-R2.1 and allows for easy updating in the future by
simply modifying the 'supportedVersions' field in the plugin.yml.

Screenshot of the new warning message: http://i.imgur.com/Ha1JqF7.png

Signed-off-by: Steven Downer <grinch@outlook.com>