Fix an issue where loading power_assert on CRuby 2.7 caused unintended errors

[k-tsj]

CRuby 2.7 has a bug that makes the following code fail even though it shouldn't.

module M
  refine Class do
    def new
    end
  end
end

class C
end

C.private_class_method(:new) #=> undefined method `new' for class `#<Class:C>'

We fix the issue where loading lib/power_assert/enable_tracepoint_events.rb during power_assert loading caused it to happen in user code.

ref:
#61