Updated to Rubocop 0.51.0.
Updated to Rubocop 0.51.0.

[Changes](http://bit.ly/2hlepWS).

There is a bug with Rubocop 0.51.0 where the `Style/CommentedKeyword`
cop raises false positives but can be ignored until the next release of
Rubocop is issued.

bkuhlmann/lineage