Added negative_failure_message function

If you try to use a negative matcher like

    expect{subject.perform(12)}.not_to instrument('offers_loader.tender_offers_loaded')

an error occur:

    undefined method `negative_failure_message' for #<RSpec::Instrumentation::Matcher::InstrumentMatcher:0x0000010a8a7b58>

This pull request adds the missing method.