Bundler is rather slow at installing new/updated gems when someone else has committed changes to the Gemfile.lock. I decided to find a quicker way. I think this is quicker, it gives me progress output so it feels quicker anyway.
$ bundle check | egrep "[a-zA-Z0-9_-]+ \([0-9.]+\)" | sed -Ee \
's/^.* ([a-zA-Z0-9_-]+) \(([0-9.]+)\).*$/\1 -v="\2"/' | xargs -L1 gem install
Fetching: SystemTimer-1.2.3.gem (100%)
Building native extensions. This could take a while...
Successfully installed SystemTimer-1.2.3
1 gem installed
Fetching: net-ssh-2.1.4.gem (100%)