export NVM_DIR in profile before sourcing nvm.sh

As there are still issues with directory discovery (#394), I suggest we export the `NVM_DIR` variable before sourcing.

With my changes the snippet added to the profile looks like this:
```sh
export NVM_DIR="/Users/koenpunt/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh"  # This loads nvm
```