bitrise (https://www.bitrise.io) is the best that I've used so far for building iOS apps. Its elite package is also really fast if you can afford it. I've used travis (slow, 45 min build times) and gitlab-ci with a dedicated mac pro runner and bitrise wins hands down for speed and simplicity. Also don't forget fastlane (https://fastlane.tools). Most of my build configuration is in fastlane (for iOS and Android) and bitrise just triggers fastlane via the script step. Good luck!