add missing `RAILS_ENV=production` bit to bundle command

@maverik Actually, for changes that doesn't modify the codebase itself, you can skip the build entirely by including [ci skip] in the commit message. The failing builds are not a consequence of your change since you only updated the CONTRIBUTING.md file so I'd say we can merge this. You can also retry the failed builds if you'd like.

Reassigned to @rspeicher

Added ~15078 label

Status changed to merged

mentioned in commit 01439ab9

@maverik Thanks!