Silence RuboCop's LineLength cop

This cop started reporting warnings again after version 0.25.0 of
Rubocop changed the name from `Style/LineLength` to
`Metrics/LineLength`.

Change-Id: I100a1ef276adc89543f8f1fcf3865007c6e5802b
Reviewed-on: http://gerrit.causes.com/41902
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>