Assorted minor updates

[jeffreyc]

@HawaiianSpork please review?

• Tweaked the language in README.md and swagger-diff.gemspec to mention the Open API Initiative
• Reformatted the description in the gemspec to eliminate embedded newlines
• Bumped the Travis Ruby matrix versions
• Bumped the dev Ruby version to 2.3.1
• Bumped the dev RuboCop version to 0.40 and fixed the 5 existing violations
• Bumped the dev WebMock version to 2.0
• Bumped the dev Rake version to 11.1

[HawaiianSpork]

LGTM