Update to notify_release_stages for env var default

[fire015]

Convert env var BUGSNAG_NOTIFY_RELEASE_STAGES to an array and allow comma separation as you can't set an array in an env file.

[GrahamCampbell]

Thanks. Could we get it more concise though? Surely this would do the same:

explode(',', env('BUGSNAG_NOTIFY_RELEASE_STAGES', '')) ?: null,

[GrahamCampbell]

Ping @kattrali.

[fire015]

No that won't evaluate to null - https://3v4l.org/VoHsO

[GrahamCampbell]

We should probably use the same exploding method we use for no in the proxy settings.

[GrahamCampbell]

Thanks, for the PR. I'll take care of this.