Handle calls to failure_slash_error_line[s].
Handle calls to failure_slash_error_line[s].

failure_slash_error_line is currently not defined in rspec-core
(ref https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/formatters/exception_presenter.rb).
This change uses it or failure_slash_error_lines if defined.

This fix addresses a current breakage, but since the code currently
monkeypatches private methods, it may still be brittle.

uroesch/serverspec,sergueik/serverspec,mizzy/serverspec,serverspec/serverspec,ihassin/serverspec,minimum2scp/serverspec