Use BUGSNAG_RELEASE_STAGE environment variable to set the release stage

[imjoehaines]

Goal

Previously we only read BUGSNAG_RELEASE_STAGE in the Rails integration and only read RACK_ENV in Rack apps, so apps using neither of those integrations have to set the release stage manually

Now we will use the 'BUGSNAG_RELEASE_STAGE' environment variable if it's set and fallback to the Rails env/RACK_ENV if those integrations are loaded

Linked issues

Fixes #421

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[kattrali]

code changes: -8 +1

best kind of new feature