Update nvm

[jackboberg]

installed here

Current version: 0.31.1. I want to clear messages like this:

gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR
gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR
gyp WARN download NVM_NODEJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use NODEJS_ORG_MIRROR

[theworkflow]

This message comes from node-gyp. However, an update to nvm is still needed.

[jackboberg]

we should make LTS the default version if not specified, rather than latest:

https://github.com/creationix/nvm#long-term-support