Fix config loading in shell

Config files from the command line would expect the wrong format:

    {app1, [...]}.
    {app2, [...]}.

Instead of the correct sys.config format:

    [{app1, [...]},
     {app2, [...]}]

Not supported yet: the recursive file references documented in
http://www.erlang.org/doc/man/config.html

src/rebar_prv_shell.erl

@@ -237,4 +237,7 @@ find_config_relx(State) ->
 consult_config(State, Filename) ->
     Fullpath = filename:join(rebar_dir:root_dir(State), Filename),
     ?DEBUG("Loading configuration from ~p", [Fullpath]),
-    rebar_file_utils:try_consult(Fullpath).
+    case rebar_file_utils:try_consult(Fullpath) of
+        [T] -> T;
+        [] -> []
+    end.