Restore old reltool vsn check

reltool is missing an app resource file before R14A.
Therefore, restore the old check and add TODO.

src/rebar_reltool.erl

@@ -81,20 +81,21 @@ clean(Config, ReltoolFile) ->
 %% ===================================================================
 
 check_vsn() ->
-    _ = application:load(reltool),
-    ReltoolVsn =
-        case lists:keysearch(reltool, 1, application:loaded_applications()) of
-            {value, {_, _, V}} ->
-                V;
-            _ ->
-                ""
-        end,
-    case ReltoolVsn < "0.5.2" of
-        true ->
-            ?ABORT("Reltool support requires at least reltool-0.5.2; "
-                   "this VM is using ~s\n", [ReltoolVsn]);
-        false ->
-            ok
+    %% TODO: use application:load and application:get_key once we require
+    %%       R14A or newer. There's no reltool.app before R14A.
+    case code:lib_dir(reltool) of
+        {error, bad_name} ->
+            ?ABORT("Reltool support requires the reltool application "
+                   "to be installed!", []);
+        Path ->
+            ReltoolVsn = filename:basename(Path),
+            case ReltoolVsn < "reltool-0.5.2" of
+                true ->
+                    ?ABORT("Reltool support requires at least reltool-0.5.2; "
+                           "this VM is using ~s\n", [ReltoolVsn]);
+                false ->
+                    ok
+            end
     end.
 
 process_overlay(ReltoolConfig) ->