insert dns-prefetch for safari

[user_agent_matcher.cc:218](https://github.com/pagespeed/mod_pagespeed/blob/master/pagespeed/kernel/http/user_agent_matcher.cc#L218) has:

```
/ TODO(rahulbansal): We haven't added Safari here since it supports dns
// prefetch only from 5.0.1 which causes the wildcard to be a bit messy.
```

Since Safari is now to version 9, we should fix this.
