Move require "jekyll/drops/drop" to "jekyll.rb"

Linux does not read files in alphanumeric order, this can lead to
Jekyll drops not working on Linux because the assumption here is that
the collection drop will be required first.

lib/jekyll.rb

@@ -169,6 +169,7 @@ module Jekyll
   end
 end
 
+require "jekyll/drops/drop"
 require_all 'jekyll/commands'
 require_all 'jekyll/converters'
 require_all 'jekyll/converters/markdown'

lib/jekyll/drops/collection_drop.rb

@@ -1,5 +1,4 @@
 # encoding: UTF-8
-require "jekyll/drops/drop"
 
 module Jekyll
   module Drops