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.
Merge pull request #4 from rwjblue/ensure-windows-works

Do not execute paths in node_modules for bower install.