Add rebar.hrl R13B04 TODO

We have to rename ?FAIL to ?ABORT once we require at least R13B04 for
building rebar. Macros with different arity were not supported by the
compiler before 13B04.

include/rebar.hrl

@@ -1,3 +1,6 @@
+%% TODO: rename FAIL to ABORT once we require at least R13B04 for
+%% building rebar. Macros with different arity were not supported by the
+%% compiler before 13B04.
 -define(FAIL, rebar_utils:abort()).
 -define(ABORT(Str, Args), rebar_utils:abort(Str, Args)).