We're not on GitHub (to my dismay) but my company does use Atlassian Stash, and they've been mimicking GitHub more and more with each update.

Our development process does include pull requests, and code review is mandatory (with the number of reviewers configurable at some level), as is a successful build from Jenkins. Stash's UI will not permit a merge operation without satisfying those criteria. There's also a restriction on the master branch so the sneaky commit isn't even possible.