Add helm to allowable completion systems

I've found this to be better than using `'default` as the completion-system with helm enabled, which shows `*helm-mode-ad-Orig-projectile-find-file*` instead of `*Helm Completions*`. It's also more obvious: I only just considered trying setting the system to `'default`. It's often faster than `'helm-projectile` in large projects as well.