Prioritize PARALLEL_RAILS_ENV over the standard

[mfilipe]

Based on the debate on #970, makes ParallelTests::Tasks.rails_env be more dynamic being able to run on different Rails environments.

The change prioritizes the PARALLEL_RAILS_ENV value, if it isn't set, it defaults to test.

Checklist

• Feature branch is up-to-date with master (if not - rebase it).
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• Update Readme.md when cli options are changed

[grosser]

let's make this a non-breaking change:

Suggested change

	ENV['PARALLEL_RAILS_ENV'] || ENV['RAILS_ENV'] || 'test'
	ENV['PARALLEL_RAILS_ENV'] || 'test'

RAILS_ENV could be get by running rake environment parallel:prepare

[mfilipe]

Hum, the only option to set a different environment will be using PARALLEL_RAILS_ENV? RAILS_ENV will be discarded.

[grosser]

yeah or we reopen #776

[mfilipe]

Hum, TBH I prefer the current solution implemented in this PR, but you are the parallel_tests maintainer (have a better overview of the gem project than me), so I can accept your suggestion and change the docs.

Probably I will need to pass PARALLEL_RAILS_ENV and RAILS_ENV in some situations, but at last it attends to my requirements (makes ParallelTests::Tasks.rails_env dynamic).

[grosser]

using RAILS_ENV looks like a good solution, but I'm pretty sure it would open up old gotchas again
and given that it has been so stable with the current solution I'm rather reluctant to risk that :D

[mfilipe]

Done c3b0bbf

[grosser]

5.4.0 🎉

fyi added the rails-env test back + fixed that it did not unset the env change