convert ppa:username/reponame to apt_line_uri

accept `ppa:launchpad-username/reponame` style with ppa checking methods
refactor #to_apt_line_uri

use String#sub to replace /^ppa:/ to ''
simplify.
Merge pull request #121 from cosmo0920/convert-apt-line

convert ppa:username/reponame to apt_line_uri