Bump to jQuery 3.3.1

[coliff]

• This is a question about using the theme.
• This is a feature request or enhancement.
• I believe this to be a bug with the theme.
  • I am using the latest version of the theme.
  • I have updated all gems with bundle update.
  • I have tested locally with bundle exec jekyll build.

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REF: http://blog.jquery.com/2018/01/19/jquery-3-3-0-a-fragrant-bouquet-of-deprecations-and-is-that-a-new-feature/