Use a more common matcher name method to support other matchers

[pirj]

Previously,

    allow(true).not_to be_nil

failed with:

#<NoMethodError: undefined method `name' for #<RSpec::Matchers::BuiltIn::BeNil:0x00007f844b43a798>> with backtrace:

matcher_name is what we use in rspec-expectations https://github.com/rspec/rspec-expectations/blob/43bf64b01f8356979ffbc373b2e81d2ab1389b29/lib/rspec/matchers/built_in/base_matcher.rb#L107