Upgrade esockd library to v4.2

rebar.config

@@ -1,4 +1,4 @@
 {deps, [
-{gproc,".*",{git,"https://github.com/uwiger/gproc",""}},{lager,".*",{git,"https://github.com/basho/lager","master"}},{esockd,".*",{git,"https://github.com/emqtt/esockd","emq20"}},{mochiweb,".*",{git,"https://github.com/emqtt/mochiweb",""}},{lager_syslog,".*",{git,"https://github.com/basho/lager_syslog",""}},{pbkdf2,".*",{git,"https://github.com/comtihon/erlang-pbkdf2.git","2.0.0"}}
+{gproc,".*",{git,"https://github.com/uwiger/gproc",""}},{lager,".*",{git,"https://github.com/basho/lager","master"}},{esockd,".*",{git,"https://github.com/emqtt/esockd","v4.2"}},{mochiweb,".*",{git,"https://github.com/emqtt/mochiweb",""}},{lager_syslog,".*",{git,"https://github.com/basho/lager_syslog",""}},{pbkdf2,".*",{git,"https://github.com/comtihon/erlang-pbkdf2.git","2.0.0"}}
 ]}.
 {erl_opts, [{parse_transform,lager_transform}]}.