Fix bugs

[yukihirop]

Summary

Fix bugs

• [BUG] A cute pet store syndrome 🐈 ([BUG] A cute pet store syndrome 🐈 #136)
• Modify gemspec files fields (Modify gemspec files fields #140)
• [BUG] Change default namespace ([BUG] Change default namespace #141)
• [BUG] R2OAS.logger.level does not work ([Bug] R2OAS.logger.level does not work #143)

rspec

===== Rspec for All Support Ruby Result =====
ruby-2.3.3: 0
ruby-2.4.2: 0
ruby-2.5.8: 0
ruby-2.6.6: 0
ruby-2.7.1: 0
=============================================

rubocop

$ be rubocop
Inspecting 165 files
.....................................................................................................................................................................

165 files inspected, no offenses detected

[yukihirop]

LGTM