Fix compile error: use diskv.WriteStream, not old diskv.WriteAndSync method

This PR fixes a compile error when building httpcache against the current version of `github.com/peterbourgon/diskv`:

```
httpcache$ go build ./...
# github.com/gregjones/httpcache/diskcache
diskcache/diskcache.go:29: c.d.WriteAndSync undefined (type *diskv.Diskv has no field or method WriteAndSync)
```

diskv.WriteAndSync was removed in github.com/peterbourgon/diskv commit
e8bbf390fbf24be1df0f5d62d30591a32ab158c5:
https://github.com/peterbourgon/diskv/commit/e8bbf390fbf24be1df0f5d62d30591a32ab158c5#L2L96.