fix: new domain for github raw data + follow redirect to avoid further issues

Since last saturday, holepicker checks systematically fail to download the vulnerability JSON file.
This is because the domain used to host raw github files changed from raw.github.com to raw.githubuserdata.com, with a permanent redirect.

In order to avoid this problem, I made the two following changes.
* First, I made sure that the online database URL was the new one.
* Second, to avoid this problem if the domain changes again, I changed the library used by the http_get method from Net::HTTP to OpenUri, which automatically follows redirects.