Relax version and add specs for Rational#marshal_dump

Removed 2.0 specs from https://github.com/rubyspec/rubyspec/commit/df824339af21e73c6655c7d90924c3d09e4c1daa.
But it is defined on CRuby 2.0 with private visibility.
Fix Rational#marshal_dump for 2.0
Relax version and add specs for Complex#marshal_dump

Removed 2.0 specs from https://github.com/rubyspec/rubyspec/commit/df824339af21e73c6655c7d90924c3d09e4c1daa.
But it is defined on CRuby 2.0 with private visibility.
Fix Complex#marshal_dump for 2.0
Add specs for Time#_dump
Fix Time#_dump for 2.0
Fix spec context for Time._load
Add specs for Time._load

Currently, have_public_method matcher is not in mspec.
Fix Time._load for 2.0
Add specs on Marshal.load for loading Rational,Complex,Time
Fix Marshal.{load,dump} for 2.0