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'.
Merge pull request #124 from justenwalker/bug-19792

Fix #19792 - cryptic error when missing deep_merge gem