Unbreak boxen-git-credential on 10.8 systems

PR https://github.com/boxen/our-boxen/pull/448 uses Ruby's Encoding class to help sudo preserve environment variables. However this breaks on OS X 10.8 as it ships with Ruby 1.8.7, which doesn't have the encoding class available: https://github.com/boxen/our-boxen/issues/474

This adds a quick conditional to ensure RUBY_VERSION > 1.9 before attempting to set Encoding, to allow this to work on >= OS X 10.8.