Updates to support Ruby 2.0, 2.1 and Rubinius 2.2

This updates Ardb to support Ruby 2.0, 2.1 and Rubinius 2.2. This
didn't require any major changes to Ardb's source. It only
required adding the Rubinius `ruby-sl` to the Gemfile.

This also changes the default `ARDB_DB_FILE` environment variable
value to expect the current working directory. This is related to
supporting Ruby 2.0 and 2.1 becasue the working directory is no
longer automatically added to `LOAD_PATHS`. Thus, the working
directory needs to be added to the default file path or the
default is basically useless. This still works for Ruby 1.8.

@kellyredding - The default `ARDB_DB_FILE` value has annoyed me when using Ardb in a non-1.8.7 environment. I'm not 100% sure that adding `.` to the beginning of the default path is how we want to handle this. A few other solutions we could go with:

* Use `ENV['PWD']` or `Dir.pwd` instead of `.`. This might be a more flexible choice than relying on `.` working.
* Leave `ARDB_DB_FILE` alone and update Ardb's CLI to add the the working directory to the `LOAD_PATH`. This has the benefit of requiring the db file relative to the load paths, which is most likely how it would be required in the system. It's a bigger change, but relatively all of these are small changes.

Let me know what you think.