Upgrade to Ruby 2.0 [#75219]

This should work for Ruby 1.9.3 and 2.0, with MySQL or Oracle.

Under Ruby 2.1.2 there are still some issues:
* https://github.com/tablexi/nucore-open/blob/master/spec/controllers/general_reports_controller_spec.rb#L243 makes an explicit check for `total_cost` to be a `Float` but it's a `BigDecimal` under 2.1. I'm thinking this test could go away.
* There are several spec failures in https://github.com/tablexi/nucore-open/blob/master/spec/models/instrument_price_policy/matrix_spec.rb and https://github.com/tablexi/nucore-open/blob/master/spec/models/instrument_price_policy_calculations_spec.rb along the lines of expecting `120` but getting a `119.9999999` `BigDecimal`.