Add arb to the list of known ruby extensions

[deivid-rodriguez]

Reference: https://github.com/activeadmin/arbre/blob/2968b4a8a4fdbe23e04d1e5e90b9dce4759e6d3c/docs/index.md

This is somewhat common due to activeadmin's popularity.

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[rrosenblum]

Seems reasonable to me.