Travis CI configured; upgrades RubyGems version.

The current version of ZenTest breaks on the current verison of
RubyGems that ships with Ruby 1.9.3 on Travis.

This Travis config causes a `gem update --system`.

    $ travis-lint
    Hooray, lazy_high_charts/.travis.yml seems to be solid!

See:
http://about.travis-ci.org/docs/user/languages/ruby/#Upgrading-RubyGems