clean old bundler binstubs on load

[hone]

It looks like there's a bug with bundle clean, where it doesn't clean up binstubs that aren't used. This patch removes the vendor/bundle/bin directory on cache load. Bundler will regenerate the binstubs even if no new gems need to be installed.

[schneems]

Internal support reference https://heroku.support/764500

The broad issue was the customer used to have the heroku gem in their gemfile which generated a binstub. They then later added the heroku CLI buildpack which should give them access to a heroku command, however the binstub from the heroku gem still took priority, even once the gem was removed from the Gemfile and re-deployed.

I'm going to try to reproduce this behavior with a dockerfile so I can report to bundler and we can get it fixed upstream as well.

[schneems]

Waiting on CI to have a successful run, also re-visiting #890 to help with making progress on the flappy tests.

[schneems]

Merging in, will let it sit on master for a bit before deploying.

Also, I'm realizing we didn't do a changelog. I'll make another PR to add one, but I would LOVE a bot that checked for the presence of a changelog entry.

[schneems]

I deployed this, you should be able to use it now.