Default to current hostname instead of `localhost`

If `process.env.EMBER_CLI_INJECT_LIVE_RELOAD_HOST` this script will be injected, which takes the current hostname of the page and uses it for serving `livereload.js`.

```html
<script type="text/javascript">
  (function(d, l, p, s) {
    s = d.createElement("script");
    s.type = "text/javascript";
    s.async = true;
    s.src = l.protocol + "//"
      + l.hostname     + ":"
      + p
      + "/livereload.js?snipver=1";

    d.getElementsByTagName("head")[0].appendChild(s);
  })(document, document.location, 12345);
</script>
```