New rule - DetectFollowedByNil

The Rails #present? check is broken.  It will never fire unless the issue is on line 3 of a file, because Object#present? calls
Object#nil? on line 3 of the core extensions file.
Replace Tracepoint with delegators

I wasn't able to get Tracepoints to work for the cases I needed.  For
example, Array#size doesn't trigger a Tracepoint callback.

So I'm trying another technique that Evan Phoenix suggested - adding
per-object-instance overrides based on what the rules want to check.