Ignore the vendor directory
use the npm global bin path

This should fix problems on travis-ci.  running `pa11y` failed, because it for some reason, didn't know where pa11y was installed.  the bin path is something like: /home/travis/.nvm/v0.10.25/lib/node_modules/pa11y

 by using `npm bin -g` we will not be subject to problems when that patch changes (for example, a different version of nvm is installed, etc)