Fix default gem key and cert locations

Currently the default gem key and cert locations are advertised like `~/.gem/gem-private_key.pem` (like [in the stdlib docs](http://ruby-doc.org/stdlib-2.0/libdoc/rubygems/rdoc/Gem/Security.html)) but the code looks at `~/gem-private_key.pem`. This updates all references to point inside the `~/.gem` directory, and extracts said references into `defaults.rb` to make it easier to update later.