Reintroduce ActiveRecord guard clause
Reintroduce ActiveRecord guard clause

If ActiveRecord has not been defined, loading ActiveRecordSubscriber
will fail because of its call to ActiveRecord::Base.

Added the guard clause to `lib/airbrake/rails.rb` instead of to
`airbrake/rails/active_record_subscriber`, since that class name implies
ActiveRecord is already available.

See:
https://github.com/airbrake/airbrake/blob/b21502812f7da58a8a3c508108c6a574cf1659f0/lib/airbrake/rails/active_record_subscriber.rb#L42
https://github.com/airbrake/airbrake/commit/4e5628f680aede2aeca5e422ccf54fca38fde8d8#diff-e09b896e9abbe09861d7846819a7bd56L32

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