rubocop 0.43+ is recommending the safe navigation operator from Ruby 2.3 be used in a number of sources files when running on Ruby 2.3, e.g.

$ be rubocop lib/ws_proxy.rb
Inspecting 1 file
C

Offenses:

lib/ws_proxy.rb:33:5: C: Use safe navigation (&.) instead of checking if an object exists before calling the method.
    socket.close unless socket.nil?
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected

It should be configured for Ruby 2.0 compatibility.