Modify mkapp to work on os x

Without `-e` silly os x will complain about labels.

```bash
+ '[' /Users/brettk/fun/psycho_app/erlang.mk '!=' /Users/brettk/fun/psycho_app/erlang.mk ']'
+ sed -i s/app_id/psycho_app/ /Users/brettk/fun/psycho_app/erlang.mk
sed: 1: "/Users/brettk/fun/psych ...": undefined label 'rettk/fun/psycho_app/erlang.mk'
```