Merge concrete's stuff into rebar config at the end.

Fixes issues with things like adding the lager_transform to rebar.config

## Before 

```
➜ notifier git:(jd/ct) ✗ rebar compile
WARN:  Missing plugins: [rebar_lock_deps_plugin]
WARN:  Missing plugins: [rebar_lock_deps_plugin]
==> edown (compile)
WARN:  Missing plugins: [rebar_lock_deps_plugin]
==> rebar_lock_deps_plugin (compile)
Compiling /Users/joe/dev/opscode/notifier/deps/rebar_lock_deps_plugin/src/rldp_util.erl failed:
/Users/joe/dev/opscode/notifier/deps/rebar_lock_deps_plugin/src/rldp_util.erl:none: undefined parse transform 'lager_transform'
ERROR: compile failed while processing /Users/joe/dev/opscode/notifier/deps/rebar_lock_deps_plugin: rebar_abort
```

## After
```
➜ notifier git:(jd/ct) ✗ rebar compile
==> goldrush (compile)
==> lager (compile)
==> rabbit_common (compile)
==> amqp_client (compile)
==> ej (compile)
==> envy (compile)
==> eper (compile)
==> ibrowse (compile)
==> jiffy (compile)
==> meck (compile)
==> rebar_lock_deps_plugin (compile)
==> edown (compile)
==> notifier (compile)
➜ notifier git:(jd/ct) ✗ 
```

It's great!