Bump nokogiri, loofah, and rack gems for security updates

loofah: CVE-2018-16468: https://github.com/flavorjones/loofah/issues/154

nokogiri: CVE-2018-14404 and CVE-2018-14567 https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md

rack: CVE-2018-16471 https://github.com/rack/rack/commit/e5d58031b766e49687157b45edab1b8457d972bd

i18n: https://github.com/svenfuchs/i18n/releases

concurrent-ruby: https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md