make ling optional

rebar.config

@@ -1,3 +1,3 @@
 {deps_dir,"deps"}.
-{deps, [{ling, ".*", {git, "git://github.com/proger/ling", {tag, "osx-again"}}},
+{deps, [%{ling, ".*", {git, "git://github.com/proger/ling", {tag, "osx-again"}}},
         {sh, ".*",   {git, "git://github.com/synrc/sh",       {tag, "1.4"}}}]}.