Do not execute paths in node_modules for bower install.

When using the `scripts` section of your `package.json` by default it adds all binaries that are in your deps and devDeps to your path. This means that `bower` is available without having to specify `node_modules/foo/bar/bin/bar`.

Also, if npm can dedupe the requirements it will. This means that the `bower` that we are supposed to use may *not* be in this projects node_modules folder.