Check for RSpec::Core before loading RSpec helpers

`paper_trail` currently breaks during initialisation whenever `rspec-mocks` is used with another test framework (eg: Minitest).

This is a minor tweak so that `paper_trail` checks that the `RSpec::Core` module is defined before trying to load the RSpec test helpers (`RSpec::Core` will be defined if either the `rspec` gem or it's dependency `rspec-core` is loaded).