How to break Travis CI build it Appium/Mocha tests fail?

I have a Travis CI project which builds an iOS app then starts Appium and runs tests with Appium/Mocha. The problem is that even though the Mocha tests fail and throw exception, the shell script which ran them via Gulp still exits with 0 and the build is deemed passing.

How can I make the build break/fail when the Mocha tests fail?