Add protobuf 2.5.0 download to gitian release process
Qt 4.8.3 r2 in gitian and release process

Also needed to add `unzip` package for the added unzip command for
OpenSSL in the Qt build.
Merge pull request #2977 from laanwj/2013_09_release_process_protobuf

Add protobuf 2.5.0 download to gitian release process