print warning when removing Gemfile.lock generated on windows

Removing `Gemfile.lock` should print a warning. It can result in unexpected versions of gems being used in your deployed app.

I'd almost argue that this breaks the `Gemfile.lock` contract and deserves an error, but I think a warning is a good start. 

This is well documented [here](https://github.com/heroku/heroku-buildpack-ruby#bundler) but not readily apparent on deploy or looking at your logs.

Workflow case: Windows user removes a gem only required in development, `Gemfile.lock` is regenerated with `PLATFORMS` as Windows. Github hides `Gemfile.lock` changes by default in the pull request and the changes get merged. On deploy, Heroku removes and ignores the locked version of your gems, all of a sudden you're using https://github.com/drapergem/draper v1.0 in production and is blowing up because of API changes from 0.18, which you were locked to originally.