Removed non-1.8 specs from Marshal.load specs. See be9a6d5
Removed ruby_version guards from Marshal.load specs. See be9a6d5
Added new Marshal.load specs for objects with user-defined _dumps

These specs were added after rbx's Marshal broke in an app that had
defined its own _dump and _load methods upon an object -- see
UserDefinedImmediate in marshal/fixtures/marshal_data.rb.
Fix Marshal for objects whose _dump returns an immediate.

specs and context behind this change: 77f88aeb