Fix 'must be purged before loading' warnings

```
=ERROR REPORT==== 12-Mar-2013::16:12:14 ===
Module lager_console_backend must be purged before loading


=ERROR REPORT==== 12-Mar-2013::16:12:14 ===
Loading of /home/andrew/lager/.eunit/lager_console_backend.beam failed: not_purged
  All 141 tests passed.
ERROR: Cover analyze failed for lager_console_backend: {not_cover_compiled,
                                                        lager_console_backend} "/home/andrew/lager/.eunit/lager_console_backend.beam"
ERROR: Cover analyze failed for lager_crash_backend: {not_cover_compiled,
                                                      lager_crash_backend} "/home/andrew/lager/.eunit/lager_crash_backend.beam"
ERROR: Cover analyze failed for lager_test_backend: {not_cover_compiled,
                                                     lager_test_backend} "/home/andrew/lager/.eunit/lager_test_backend.beam"
```