Land rapid7#6448, set the high-level require to metasploit/credential

Author: Brent Cook

lib/metasploit/framework.rb

@@ -9,8 +9,7 @@
 require 'bcrypt'
 require 'json'
 require 'msgpack'
-require 'metasploit/concern'
-require 'metasploit/model'
+require 'metasploit/credential'
 require 'nokogiri'
 require 'packetfu'
 # railties has not autorequire defined