Updated URL in README.md

Looks like github changed the raw URL path.
```
$ curl -L https://raw.github.com/napalm255/powerbash/master/powerbash.sh
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>400 Bad Request</title>
</head><body>
<h1>Bad Request</h1>
<p>Your browser sent a request that this server could not understand.<br />
</p>
</body></html>
```