add .travis.yml
Add Gemfile
Add travis-ci build badge and code climate.  Update tested ruby versions.
Minor changes to Travis config

Just like in Sequel, use a special .travis.gemfile for Travis,
don't have a general Gemfile.  Don't have a lock file for the
gemfile, it's not necessary.  Use the same rake/rspec version
constraints that Sequel uses for testing.

Only test on rbx-2, not both rbx-2.1 and rbx-2.2.  I don't think
it makes sense to test two versions of rbx.  Allow rbx to fail,
but not any of the other ruby versions to, since rbx on Travis
often fails for reasons unrelated to the code being tested.

Remove banners for Travis and Code Climate from the README. In
general if they are passing it means nothing, and failures are
often unrelated to the code being tested.  That's true for Travis
in my experience.  For Code Climate, I just don't see the point.
Note why we don't handle - or . separated dates in the README
Deal with new rake and old rdoc in Rakefile

Newer versions of rake have a rake/rdoctask that raises a
RuntimeError if you try to load it.  This is a poor design
decision (raising a LoadError would be better), but that's life.

In any case, if rdoc/task doesn't exist and rake/rdoctask raises
an error, don't create an rdoc task, and have the rake actually
run instead of exiting with an error.