fix download url for hamsterdb 2.1.7

This PR points the hamsterdb formula to the correct download location. The old url results in a 404:

```shell
➜  ~ git:(master) ✗ brew install hamsterdb
==> Downloading http://hamsterdb.com/dl/hamsterdb-2.1.7.tar.gz

curl: (22) The requested URL returned error: 404 Not Found
Error: Failed to download resource "hamsterdb"
Download failed: http://hamsterdb.com/dl/hamsterdb-2.1.7.tar.gz
```

I found the new url at http://hamsterdb.com/download/ .