Let Commander handle bad args after printing of deprecation warning. Fixes #1040.

lib/jekyll/deprecator.rb

@@ -20,14 +20,12 @@ module Jekyll
       if args.size > 0 && args.first =~ /^--/ && !%w[--help --version].include?(args.first)
         Jekyll::Logger.error "Deprecation:", "Jekyll now uses subcommands instead of just \
                             switches. Run `jekyll help' to find out more."
-        exit(1)
       end
     end
 
     def self.deprecation_message(args, deprecated_argument, message)
       if args.include?(deprecated_argument)
         Jekyll::Logger.error "Deprecation:", message
-        exit(1)
       end
     end
   end