Change Node.js executable to be a variable

Node.js executable can be a variable with this change since the actual name of the executable varies with the distribution. Newer Debian distributions - Ubuntu 14.04 LTS uses 'nodejs' instead of 'node'.
This will break in environments that have node in other locations than `/usr/bin/node`. Please do the check the other way around, i.e. look in known locations for `nodejs`, and fall back to `node` as default.