Fixed command to install npm

Fixed command to install npm. When I ran the command:
```curl http://npmjs.org/install.sh | sh```
I got an error: 'sh: 2: Syntax error: newline unexpected'.

The reason for this can be found at: https://github.com/isaacs/npm/issues/2675
The solution is to use https (instead of http) in the URL. This is what this pull request does.