Add merl dep also when building slex files.

rebar-slex.config

@@ -1,7 +1,8 @@
 %% -*- mode: erlang -*-
 
 {deps,
- [{slex, ".*", {git, "git://github.com/erlydtl/slex.git", {branch, "master"}}}
+ [{slex, ".*", {git, "git://github.com/erlydtl/slex.git", {branch, "master"}}},
+  {merl, ".*", {git, "git://github.com/erlydtl/merl.git", {branch, "erlydtl"}}, [raw]}
  ]
 }.