Fix #19792 - cryptic error when missing deep_merge gem

The configuration loader will try to pre-emptively load 'deep_merge' if
the 'deep' or 'deeper' options were given to :merge_behavior.

If the load fails, it will warn the user and revert back to 'native'.

```
WARN: Tue Mar 26 18:02:33 -0400 2013: Ignoring configured merge_behavior
WARN: Tue Mar 26 18:02:33 -0400 2013: Must have 'deep_merge' gem installed.
```