require files in test/setup.rb through bundler.
remove Tracer#{events,targets}=

'events' have become a constant and reduced down to two elements:
"call" and "return". "call" is an event that happens when a ruby method
is called and "return" is an event that happens when a ruby method
returns.

'targets' are passed directly to IRWebmachine::Tracer.new() and cannot
be modified through a setter anymore.