Deprecate RACK_ENV in favour of APP_ENV

[svoop]

Say goodbye for the often misused RACK_ENV in favour of APP_ENV (like Sinatra does). A deprecation warning is printed if only RACK_ENV is set and Golllum has not yet reached version 6.

Related issue: #2029

[benjaminwil]

Thanks for this contribution.

I will defer to @dometto to approve and merge, as he has a clearer picture of what the 6.0.0 release looks like.

[benjaminwil]

I was surprised to learn that this string comparison works.

If Gollum was a project that had more pre-releases (i.e. 6.0.0.alpha) I would reach for a more-expressive Gem::Version comparison:

Gem::Version.new(Gollum::VERSION) < Gem::Version.new("6.0.0")

But I manually tested that this does the right thing, and it does. So looks good to me.

[svoop]

Actually, I'm not really sure whether checking for the version there is a good idea at all. Might just as well without, provided someone keeps track of such post-release TODOs.

[dometto]

I think it could be alright to leave out the version check, if we open an issue to remove the deprecation notice. Even with the check present, we'll have to remember to remove the code at some point, so it doesn't make that much of a difference I think.

So I propose that we keep the deprecation notice (without the version check) until 7.0 (because 6.0 will happen very soon when I'm back from leave) and open an issue to remind ourselves to remove the relevant code in that release.

I also learned something new about string comparisons today! 😀

[svoop]

@dometto Sorry for the delay, this slipped off my desk somehow. I've removed the version check and just kept the # TODO which mentions the lines to be removed once gollum-7 is released.

[dometto]

Thank you @svoop and sorry for the delay!

[svoop]

Easy, thanks for the merge!