Fix bug concerning nested defs in `EmptyLineBetweenDefs` cop.

Fixes #400.

Avoids the problem with nested `def`s by only checking if the `def` line is the next line from the previous method `end`. I figure that if the `def`s are on the *same* line, that's not this cop's problem. Nested `def`s can still mean that we miss some offences. This change at least removes the false positives.