add Vagrant's embedded dir to the `PATH` 

This is useful for developing vagrant plugins because some plugins rely on existing executables to be present (e.g. `curl`) which is case if they are executed in a Vagrant environment but not necessarily (especially on Windows) if they are executed in a plain Ruby environment (e.g. when developing that plugin).

Apart from that concrete use case it does not hurt to have some more unix tools available on the `PATH`.

If it turns out that it would conflict with the devkit (whose bindir is not explicitly added to the `PATH` but might be temporarily added in case of compiling native gems) we have to remove it again. (P.S.: the devkit unfortunately does not contain `curl` so it does not fix the above use case.)