Update mail_room gem

What does this MR do?

Upgrades the mail_room gem from 0.8.0 to 0.8.1.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

mail_room throws an exception if the watching script joins a undefined thread. Release 0.8.1 adds a check to ensure that the thread exists before calling join.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes omnibus-gitlab#1619 (closed).

cc: @maxlazio