Merge #7137

7137: Gemspec require relative r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

This is a follow up to #7100. Gemspec is another place where `require_relative` sounds like a win.

### What is your fix for the problem, implemented in this PR?

My fix changes bundler's gemspec to use `require_relative`, and extracts the relevant change from #4397 to also change the generated gemspec from `bundle gem`.

Co-authored-by: Miklos Fazekas <mfazekas@szemafor.com>
Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>