Full paths to subcommand source and Ruby may be given,
which is helpful in troubleshooting, particularly for
forcing a specific Ruby version.
Example usage:
```bash
$ HOMEBREW_BREW_FILE=/usr/local/bin/brew /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby /usr/local/Library/brew.rb /usr/local/bin/brew-cask.rb help
```
Previously, this would work, but throw an odd error when
returning to Homebrew. Fix: we unconditionally do not
return to Homebrew.
Fixes#4677. This change is necessitated by Homebrew's recent
switch to Ruby 2.x. These changes may be incomplete and/or
may not work well with Ruby 1.8. Travis should test Ruby 1.8
compatibility.
Found while investigating #2275, but probably not a fix, just a sensible
change. Depending on version of ruby interpreter, previous code could
cause brew-cask paths to came later than current dir "." in $LOAD_PATH,
leading to unpredictable behavior in the wild.
Roland Walker