Provide assertions accessor for minitest.

Fixes https://github.com/rspec/rspec-core/issues/1354

I don't have a great idea for integration testing this: latest `rubysl-test-unit` (in the `Gemfile`) is locked to minitest 4.7. For similar reasons, I haven't touched the `Test::Unit::Assertions` include suggested by @myronmarston ( https://github.com/rspec/rspec-core/blob/184b02c7673b61588129bad268eb0847bae5243a/lib/rspec/core/configuration.rb#L498-L500 )

I have confirmed that this fixes @rosenfeld's sample application.