Disable bundler warning when run as root

[blackxored]

I think I've finally found the right place to put this. 😹
Would you accept a pull request that disables Bundler's root warning by default?
I mean this warning:

root@5677827dcdc3:/usr/src/app# bundle
Don't run Bundler as root. Bundler can ask for sudo if it is needed, and installing your bundle as root will break this application for all non-root users on this machine.

I'd be happy to add it.

[yosifkit]

Sounds good to me! I look forward to your PR. 😄