Fix process term for sidekiq-monitoring rackups

www.agileplannerapp.com/boards/173808/cards/5827

earlier, on receiving TERM foreman terminated a single
master process, and left child processes dangling. this
caused failure to restart `rackup`s after a deploy, as
foreman restarted, and dangling `rackup`s blocked ports.

foreman 0.65.0 fixed this issue by running the command
with exec, and ensured that child processes were asked
to TERM when the master process received a TERM:

https://github.com/ddollar/foreman/pull/428

hat tip to @alext, for suggesting that we need to run
`exec bundle exec`. foreman's exec wrapping only takes
care of executing `cd` replacing the current process
and `bundle exec` will still create a child process.
Merge pull request #5 from alphagov/fix-foreman-not-killing-child-processes

Fix process term for sidekiq-monitoring rackups