Add Travis CI to check build

[erictleung]

• https://jekyllrb.com/docs/continuous-integration/
• https://github.com/gjtorikian/html-proofer
• https://docs.travis-ci.com/user/languages/ruby/#Dependency-Management
• http://stackoverflow.com/questions/2881482/what-is-a-rakefile

[erictleung]

Closed by https://github.com/erictleung/erictleung.github.io/commit/2cca6888dea8ce177ed525e6543180d0afb4af61