This should never happen in normal operation, but is possible when
running RSpec tests

See thoughtbot/factory_bot_rails#303 (comment)
and #534

Essentially this happens:
1. RSpec boots Rails
2. Rails boot process errors
3. Rails freezes middleware
4. RSpec moves on to the next test file
5. Bugsnag tries to add middleware but fails because it's frozen
6. The stacktrace blames Bugsnag for this test failure
7. goto 4

Supressing this error doesn't solve the problem as it's likely another
frozen error will happen in some other library/component, but we want
to avoid people thinking this is caused by Bugsnag. The stacktrace
does point out the actual problem, but only in the very first error,
which can get buried under the frozen errors that happen for each
test file

Convert frozen middleware error into a warning in the Rails integration

Set consistent context in delayed_job integration

Previously we read 'BUGSNAG_RELEASE_STAGE' only in the Rails
integration, so other apps had to use 'RACK_ENV' (if they run in Rack)
or set it manually in code

Now we will always use the 'BUGSNAG_RELEASE_STAGE' environment variable
if it's set

This should now happen automatically for us, so the existing MR tests
can verify that this feature works :^)

Use BUGSNAG_RELEASE_STAGE environment variable to set the release stage

This fixes a couple of issues with `app_type` — we now always use
the value set by a user (if one is set) and integrations should now
always set the correct value

For example, before this change Que, when running in a rails app,
would have an app type of 'rails'. This is because the Que integration
has to run when it's loaded and so sets its app type before Rails does

[PLAT-3643] Ensure app_type is consistently resolved

This adds runtime versions for:
- Delayed Job
- Mailman
- Shoryuken

Mailman & Shoryuken are straightforward, but Delayed Job doesn't have
a constant with its version in. Instead we can read it from the loaded
Gems

[PLAT-3702] Report runtime versions for Delayed Job, Mailman and Shoryuken

These are needed to be able to install the Gem from the built gemfile

Reduce gem size

Delay JSON serialization until delivery when using the thread queue delivery method

Pin Rubocop version more strictly