first try on faster bundle exec ruby

as per discussion in #2117

```
normal ruby:
time ruby -rbundler/setup -e 'puts "foo"' --> 2.3s

bundle exec with patch:
time bundle exec ruby -e 'puts "foo"' --> 2.9s

bundle exec without patch:
time bundle exec ruby -e 'puts "foo"' --> 4.0s
```

the time diff in case 2 is because rubygems is loaded once for the executable and once for the exec ruby run. Getting rid of it like the hub gem does might be a solution, but still kind of nasty...

I'm trying to preserve the original ARGV, so that e.g. `ruby -e ''` does not blow up, maybe there is a better way of doing this...