Change the rule about magic comments

* Move the rule on ruby 2.0+ to MUST
* Specify "どうしてもマジックコメントが必要な場合" (== working on ruby 1.9.x)
* Fix a wrong target version (en) (1.9+ to 1.9.x)
Add character encoding rule
Merge pull request #7 from makimoto/magic-comment

Change the rule about magic comments