Add missing test dependency to README.md

[hoshinotsuyoshi]

When using slack-ruby-bot/rspec, rack-test gem is needed because of this

Without this gem, I cannot pass my tests.

$ bundle exec rspec
(...snip...)
/Users/hoshino/go/src/github.com/hoshinotsuyoshi/dicebot/vendor/bundle/ruby/2.5.0/gems/slack-ruby-bot-0.10.5/lib/slack-ruby-bot/rspec.rb:5:in `require': cannot load such file -- rack/test (LoadError)

TUTORIAL.md is correct.

https://github.com/slack-ruby/slack-ruby-bot/blob/v0.10.5/TUTORIAL.md#gemfile

[dblock]

Thanks!