Fetch the newest gem when --version is given

Previously gem fetch and gem install had mismatched behavior.  Install
would pick the newest version while fetch would pick the oldest due to a
backwards version picking in the fetch command.

Now max_by is used to pick the largest version which is also more clear
for future readers.

Fixes #643
Fix host on gem push, when not passing a --host

After d3f91a011705020c4f1e601d8ddab1c924e5eb99 we start accepting the option --host on the push command
However when the --host is not provided, it is set as nil, and this was causing the following output

Enter your  credentials.
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Update history for #622
Fix warning in GemcutterUtilities tests

Parentheses were missing the test added in #622
Update version for release
Add missing "bug fixes" title to 2.0.9
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