Don’t raise an exception on SIGTERM

Looking at our Rollbar quota, the large majority of the errors we get
there come from, I believe, the recurring Heroku restart that happens.
I see no reason to report these to Rollbar, since they are expected
and, at least while using Heroku, necessary.

Luckily, looks like [Puma has an option for this][0], so it makes sense
to me to turn it on (or rather off, since we’re setting it to `false`).
[Found it][1] while doing some research on this subject, since I’ve had
a feeling I’ve seen it before.

[0]: https://www.rubydoc.info/gems/puma/Puma%2FDSL:raise_exception_on_sigterm
[1]: puma/puma#1438

config/puma.rb

@@ -6,6 +6,7 @@
 rackup      DefaultRackup
 port        ENV['PORT']     || 3000
 environment ENV['RACK_ENV'] || 'development'
+raise_exception_on_sigterm false
 
 on_worker_boot do
   # worker specific setup