Change default base URI to use opscode-omnibus-packages

Also enables SSL.

Old default was 'http://opscode-omnitruck-release.s3.amazonaws.com', some client
packages are unavailable at this source. (eg 11.4.4 on Ubuntu).

The new URI is taken from files offered from the official Opscode download page
(http://www.opscode.com/chef/install/).

`${BASE_URI}/ubuntu/11.04/x86_64/chef_11.4.4-2.ubuntu.11.04_amd64.deb` works with this change when it didn't work with the previous base URI.

I have no insight into whether this is the new official URL from Opscode, or if the old one is truly deprecated. However, this seems to work.