Fix executable not working

Need to tell the system it's dealing with a node script. This fixes both
the installed `kredits-ipfs-pinner` command as well as npx (which is
also running that command).

index.js

@@ -1,4 +1,5 @@
-const debug = require('debug')('ipfs-pinner');
+#!/usr/bin/env node
+
 const Kredits = require('kredits-contracts');
 const IpfsPinner = require('./lib/ipfs-pinner');