Add /opt/boxen/nvm/$version/bin to path via /opt/boxen/env.d/nvm.sh

This is where npm installs binaries like hem and spine, which are meant to be available once the packages are installed. The current 1.0.0 version doesn't include anything but $CWD/node_modules/.bin (apparently).

To reproduce the motivation for this patch:

1. npm install -g spine
2. cd $workdir
3. spine app sample
4. spine executable isn't found.
5. /opt/boxen/nvm/v0.8.8/bin/spine app sample
6. Now it creates the sample app.