PPA verification fix

The method ppa() is supposed to take a ppa name and skip if already installed (with `is_source` method).

1) If you pass a PPA with a `.` like the one from the example (`require.deb.ppa('ppa:chris-lea/node.js'`), it will end up as `_` on Ubuntu (and Debian).

2) The check for the PPA will never work as it's assuming the current directory being where apt look for sources `/etc/apt/sources.list.d`

This PR fixes both issues.

Feel free to comment on it.

Thank you!