Credit to postmodern for developing the auditing gem and also to RubySec for creating the ruby-advisory-db, a community maintained database of Ruby gem vulnerabilities for which bundler-audit is built on top of.
So to install this -
gem install bundler-audit
to run it, navigate to the directory where the Gemfile.lock is stored:
If the application is using a vulnerable version of a gem, the output will look like...