The default is ruby 2.0.0.
Merge pull request #14 from yasuhito/feature/ruby2.0.0

The default is ruby 2.0.0.
Enable rubocop.
Merge pull request #15 from yasuhito/feature/rubocop

Enable rubocop.
Update README.md
The default rake task kicks rspec, cucumber and rubocop.
Refactoring.
Enable Travis CI.
Add trema prerequisites for Travis.
Fix 'Missing utf-8 encoding comment' error (1.9.3)
add .
Delete graphviz.rb~
Delete link.rb~
Delete topology.rb~