Gemfile: bump rubocop to '= 0.81'
Gemfile: bump rubocop to '= 0.81'

While this is not the latest Rubocop version, it's the last one that supports
Ruby 2.3, which is our oldest Ruby that we support. Therefore we'll be on 0.81
for quite some time.

nick-desteffen/airbrake,nick-desteffen/airbrake,airbrake/airbrake,airbrake/airbrake