lib/cross_platform.rb: when using MRIv2.1, avoid to load native extension.
.travis.yml: test against MRI 2.1.1
ext/extconf.rb: avoid native extension for MRIv2.1 later
lib/interception.rb: methods depending on platform move to cross_platform.rb
lib/interception.rb: add abstract methods that's only raise NotImplementedError
Merge pull request #12 from ygotoh/refactoring

merge requeset for refactored code and backporting offer.