Bump the Ruby version to 2.0.0-p481

The Ruby version 2.0.0-p451 cannot be built on newer systems which use
readline-6.3 due to bug #9578. The Ruby version 2.0.0-p481 has the fix.
Merge pull request #37 from petrhosek/bump-ruby-to-latest

Bump the Ruby version to 2.0.0-p481