Use sqlite3-ruby 1.3.8, not 1.3.9

This fixes failing specs for rbx. For more details, see: https://github.com/sparklemotion/sqlite3-ruby/issues/122
platforms :ruby means MRI and Rubinius so we don't need :rbx

For more details on how to specify platform, see: http://bundler.io/v1.3/man/gemfile.5.html
Do not use test/unit for AR 4.1 tests