Fixed README to avoid deprecation warning

[akito19]

When I used this gem, the logs noted deprecation warning. So I fixed README to avoid being pointed out.

[Gretel] Calling `breadcrumbs` with a block has been deprecated and will be removed in Gretel version 4.0. Please use `tap` instead. Example:

 breadcrumbs(autoroot: false).tap do |links|
    if links.any?
      # process links here
    end
 end

[akito19]

The mime-types-data gem prevents bundle install from CI build in case of Rails 4+ because the gem requires Ruby 2+.
Should I wait for merging pull requests which had already been suggested to solve these problems?