v1.0.7: Add secure signature to gem

Previously 1.0.6 didn't work because RubyGems was still in
read-only mode. Since then, I changed the gem cert so the
1.0.6 release is effectively in /dev/null now.

Here's what happened:

- Add pkg/ for holding built gems on your machine
- Replace public cert with the newly generated one

With this release and moving forward, all Rain releases will
be securely signed, so you know where the code is coming from.