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.