use BasicObject as base class for Attributor::Model instead of Object

[smessier]

The issue is that 3rd-party DSLs such as Rake (pre-v10) will define methods on Object that can conflict with attribute names. Since Model does not remove pre-existing method definitions it will simply inherit from Object a method such as 'namespace'. When the 'namespace' attribute is invoked the code instead calls through to Rake v9.

Using BasicObject is tricky, of course, because it lacks useful methods like #to_s, #inspect, #methods, etc