Take third parameter for Sidekiq error_handler (#1136)

In
sidekiq/sidekiq@2870516
Sidekiq starts accepting a the Sidekiq Configuration option as a
parameter to error_handlers and marks two parameter handlers as
deprecated.  This change adds a third parameter to the handler to accept
that configuration and defaults the value to `nil` since

1. Older versions of Sidekiq will not pass a third parameter
2. Rollbar does not need to do anything with that value (yet?)

lib/rollbar/plugins/sidekiq.rb

@@ -12,7 +12,7 @@
         chain.add Rollbar::Sidekiq::ResetScope
       end
 
-      config.error_handlers << proc do |e, context|
+      config.error_handlers << proc do |e, context, _sidekiq_config = nil|
         Rollbar::Sidekiq.handle_exception(context, e)
       end
     end