Set release-candidate version
Set licenses
Improve performance of `gem outdated`

Added Gem::Specification::outdated_and_latest_version which enumerates
outdated local specifications and the latest version.  This removes
duplicate work between Gem::Specification::outdated and the
OutdatedCommand.

On 244 gems I see 154 seconds for RubyGems 2.0.x and 95 seconds for this
version.