Update nvm install script to use the one from githubusercontent.

Currently when installing nvm, ```curl https://raw.github.com/creationix/nvm/master/install.sh``` is used. However, github now redirects raw files to raw.githubusercontent.com, and the curl command does not follow the redirect, which makes the download of the nvm install script fails.

I found there are some other projects having the same issue, and the simplest solution is changing the github.com to githubusercontent.com.

For example:
https://github.com/pote/gpm/issues/30
https://github.com/prose/prose/issues/697