Upgrade Pipenv to 2018.11.26

[lsmag]

Hi! This is related to #786

We're using Pipenv at my team but we've had problems with the version this buildpack pinned, 2018.5.18. In the end we leveraged the compile hooks to install a different version, because this was a blocker and I didn't know a better way to do it... aside from upgrading the version upstream, which is what this PR is about.

This upgrades Pipenv to 2018.11.26, but also allows the variable to be overriden, similar to how it is done in other sections of the code.

The only thing I'm still confused about is how to test the overriding part. I've read test/run a bit, and I think I can create a new testOverridenPipenv function that exports the variable, then unsets it after success. Will it be enough?

[lsmag]

Oops, the CI failed with something I'm not familiar with :S https://travis-ci.org/heroku/heroku-buildpack-python/jobs/534864670

I'll need your help here to get this PR in a mergeable state, this has been blocking pipenv to be widely adopted at my company.

[jerr0328]

Looks like PRs from non-members fails (secrets aren't added in, which makes sense because you don't want a malicious PR to expose those secrets in Travis). I do hope this can get merged, it would help me since I have the same problem as #704

[lsmag]

If it helps merging faster, I can change it to export PIPENV_VERSION="${PIPENV_VERSION:-2018.5.18}" , so that current builds don't break and we're still allowed to customize Pipenv.

[lsmag]

Huh, sorry to ask again, but what's the status on this? I'm willing to put more effort myself into fixing tests and whatever else is needed, but I will need some guidance here.

This is fundamental to allow the company I work for to use Pipenv across all teams.

[CaseyFaist]

Hi there! some context:

Unfortunately, the current version of Pipenv would require users to uninstall and reinstall their local pipenv environments before being able to deploy again successfully. This is due to incompatibility with the lock files that is not autocorrected by pipenv's update function. Since this would require coordination between the release and all pipenv users, or would result in a temporarily opt-in upgrade (which, if anyone missed the communications around it, could lead to quite a lot of frustration), we're not looking at this update any time soon.

I am sorry about this frustration, however our version (2018.5.18) is also not as behind the current version (2018.11.26) as it has been since the most recent pipenv release. I'd like to see a bit more stability in the release cycle from the project before prioritizing this issue.