Only patch File on JRuby < 1.7.14

MSP-11372

JRuby 1.7.14 fixed the File.realpath bug, so Metasploit::Model::File
only really needs to exist for JRuby < 1.7.14, but since pushing that
logic into other code complicates the code, leave
Metasploit::Model::File, but just make it an alias of ::File when the
patch for JRuby < 1.7.14 is unnecessary.