Fix missing Administrate::VERSION reference

In #2479, we switched to using a dedicated `ActiveSupport::Deprecation`
instance to handle a deprecation warning. Unfortunately, in some
circumstances, we reference the `VERSION` constant when it's not been
loaded, which seems to happen when we render the deprecation warnings.

lib/administrate.rb

@@ -1,4 +1,5 @@
 require "administrate/engine"
+require "administrate/version"
 
 module Administrate
   def self.warn_of_missing_resource_class