Bundle Update: Scrivito 1.15.0, Rails 5.1.6.1, and deleting fixed gem for workaround

[yasulab]

cf. #364

This PR updates gems, but not upgrade Rails to 5.2.x because Scrivito 1.15.0 is still not available in Rails 5.2.x or higher.

Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    rails (= 5.2.2)

    scrivito (~> 1.13.0) was resolved to 1.13.0, which depends on
      scrivito_sdk (= 1.13.0) was resolved to 1.13.0, which depends on
        rails (< 5.2, >= 4.2)

[yasulab]

I will merge this PR when CI passed. 👀 ✅ ✨