utils/ruby.sh: simplify and fix Ruby-related logic
utils/ruby.sh: simplify and fix Ruby-related logic

General:
- Rename `unusable_ruby` to `need_vendored_ruby` (due to code changes).

**`setup-ruby-path`** function:
- Move `-z "$HOMEBREW_RUBY_PATH"` and `-n "$HOMEBREW_FORCE_VENDOR_RUBY"` checks to `need_vendored_ruby`.

**`need_vendored_ruby`** function:
- Check `HOMEBREW_FORCE_VENDOR_RUBY` first so that it's respected even
  on Macs that are "new enough" (`HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH` is set).
- Don't check `-n $HOMEBREW_RUBY_PATH` because it is implicitly tested
  by `test_ruby`.
- Remove (now-) redundant `-z $HOMEBREW_FORCE_VENDOR_RUBY` check as it is
  tested earlier in the conditional.

MikeMcQuaid/brew,sjackman/homebrew,Homebrew/brew,konqui/brew,JCount/brew,vitorgalvao/brew,nandub/brew,nandub/brew,nandub/brew,konqui/brew,MikeMcQuaid/brew,JCount/brew,claui/brew,claui/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew,Homebrew/brew,MikeMcQuaid/brew,claui/brew,EricFromCanada/brew,nandub/brew,EricFromCanada/brew,sjackman/homebrew,JCount/brew,konqui/brew,JCount/brew,vitorgalvao/brew,vitorgalvao/brew,EricFromCanada/brew,sjackman/homebrew,EricFromCanada/brew,vitorgalvao/brew,konqui/brew,claui/brew,sjackman/homebrew