Add `.nvmrc` with `lts/carbon`
Add `.nvmrc` with `lts/carbon`

This way you can just call `nvm use` in the project dir and have the
proper node version selected.

See https://github.com/creationix/nvm#nvmrc

Issue: BG-1460

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS