Check for /usr/bin/nodejs for Debian/Ubuntu based systems

I tried installing keybase, but it failed on Ubuntu with this error:

```
rdhruva@ubuntu:~$ keybase-installer
/usr/bin/env: node: No such file or directory
```

Turns out that Debian based systems install node as /usr/bin/nodejs and not /usr/bin/node (because of an already existing package called node). I suggest checking for /usr/bin/nodejs in addition to 'node'. If that is not possible, putting a note on website that debian-based linux systems should install package `nodejs-legacy` also works (this package creates a symlink `/usr/bin/node` to `/usr/bin/nodejs`.

Start by reproducing the keybase-installer failure on a Debian- or Ubuntu-based system and trace where it invokes `node`. The change is done when the installer works with `/usr/bin/nodejs`, or the website clearly documents the `nodejs-legacy` workaround; the payload names no source files or tests.

Written by the indexing model from the issue text.