AV::Digestor thread safety fixes.

This fixes potential thread safety issues introduced in 09f6fe1. The problem
with the original code was that the "recursion-stopping" `nil` values could
be seen by other threads.
Merge pull request #11664 from thedarkone/digestor-thread-safety

AV::Digestor thread safety fixes
Add retry to `gem install bundler` for Travis

This adds the retry behaviour used for other commands to the bundler installation.
Merge pull request #11672 from henrikhodne/patch-1

Add retry to `gem install bundler` for Travis
Add arel to Gemfile