chore(*): Add .nvmrc

If one uses nvm to manage Node.js versions, the .nvmrc file makes `nvm use`
switch to the version specified in .nvmrc. There are scripts that invoke
it automatically when cd'ing to directories containing .nvmrc so that you
never run build commands using a wrong Node version, see:
https://github.com/creationix/nvm/blob/v0.31.2/README.markdown#zsh