Fix CVE-2013-4363, remove regexp backtracking

The Gem::Version regexp used backtracking to validate gem versions, but
in a different way than CVE-2013-4287. This could cause excessive CPU
usage when creating Gem::Version objects including when packaging gems.
See CVE-2013-4363.txt (in this commit) for details.

See #626
Update version for release
Add CVE-2013-4363.txt to rdoc