Depend on emqtt/pbkdf2 to resovle the building errors of Travis CI

Makefile

@@ -2,7 +2,7 @@ PROJECT = emqttd
 PROJECT_DESCRIPTION = Erlang MQTT Broker
 PROJECT_VERSION = 2.1.0
 
-DEPS = goldrush gproc lager esockd mochiweb lager_syslog pbkdf2
+DEPS = goldrush gproc lager esockd mochiweb pbkdf2 lager_syslog
 
 dep_goldrush     = git https://github.com/basho/goldrush 0.1.9
 dep_gproc        = git https://github.com/uwiger/gproc
@@ -10,6 +10,7 @@ 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_mochiweb     = git https://github.com/emqtt/mochiweb
+dep_pbkdf2       = git https://github.com/emqtt/pbkdf2 2.0.1
 dep_lager_syslog = git https://github.com/basho/lager_syslog
 
 ERLC_OPTS += +'{parse_transform, lager_transform}'

rebar.config

@@ -1,4 +1,4 @@
 {deps, [
-{goldrush,".*",{git,"https://github.com/basho/goldrush","0.1.9"}},{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,"",""}}
+{goldrush,".*",{git,"https://github.com/basho/goldrush","0.1.9"}},{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",""}},{pbkdf2,".*",{git,"https://github.com/emqtt/pbkdf2","2.0.1"}},{lager_syslog,".*",{git,"https://github.com/basho/lager_syslog",""}}
 ]}.
 {erl_opts, [{parse_transform,lager_transform}]}.