Redirect legacy traffic to legacy.polyfill.io

In preparation for moving the service to the polyfill.io domain:

```
> curl -i -H 'Referer: http://www.manrepeller.com/' -H 'Host: polyfill.io' -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.111 Safari/537.36' http://localhost:3000/normalize.css

HTTP/1.1 301 Moved Permanently
Cache-Control: public, max-age=86400, stale-while-revalidate=604800, stale-if-error=604800
Location: http://legacy.polyfill.io/normalize.css
Content-Type: text/plain; charset=utf-8
Content-Length: 440
ETag: W/"1b8-751d760e"
Date: Wed, 12 Nov 2014 11:42:49 GMT
Connection: keep-alive

The hostname www.manrepeller.com is known to be a user of the previous version of polyfill.io, so for backwards compatibility we're redirecting you to legacy.polyfill.io which will handle your request using the old version of the service.  The legacy version will be discontinued in the future, so please update to the new version, and then let us know by raising an issue in our repo at https://github.com/financial-times/polyfill-service.⏎             
```
Ah, dammit, yes of course.  And we can't do that if we want any kind of decent cache perf.

I might have to come up with an alternative way of doing this.