Upgrade esockd library to v4.2

Makefile

@@ -9,7 +9,7 @@ DEPS = gproc lager esockd mochiweb lager_syslog pbkdf2
 dep_gproc        = git https://github.com/uwiger/gproc
 dep_getopt       = git https://github.com/jcomellas/getopt v0.8.2
 dep_lager        = git https://github.com/basho/lager master
-dep_esockd       = git https://github.com/emqtt/esockd emq20
+dep_esockd       = git https://github.com/emqtt/esockd v4.2
 dep_mochiweb     = git https://github.com/emqtt/mochiweb
 dep_lager_syslog = git https://github.com/basho/lager_syslog
 dep_pbkdf2       = git https://github.com/comtihon/erlang-pbkdf2.git 2.0.0