“CI” stands for continuous integration. Before new code is merged into a project, there are certain standard steps that should be taken every time. CI is when those tasks are automated, so that they happen every single time before code is merged, without a person having to remember to do them.
For instance, every time a pull request is opened, Hound automatically comments on lines that contain stylistic violations. This way, style guidelines get enforced, but the people doing the code review get to spend their time and energy on more big-picture design issues.
Travis is CI for testing: every time there is a pull request, Travis pulls down the code, runs the full test suite (I believe you can even specify multiple versions of Ruby and maybe even gem versions if you want to test compatibility!!), and reports the result on the GitHub pull request. Now, we no longer have to worry about pulling the code down locally and running the test suite ourselves before we merge code: Travis takes care of that for us.
These are both awesome, awesome tools that can save Rubyists tons of time and make our code better. What’s better, they are super easy to set up, and free for open source projects!
Try them out!