You’re right, the app should build correctly, without special instructions, just by cloning the repo. (With the occasional exception of requiring Android Studio beta versions, which I think is a reasonable requirement for anyone cloning the repo. It may be mildly annoying, but I only require AS betas when it provides a meaningful velocity boost on my end.)
Anyway, the google-services.json issue will be fixed in the next couple days, but for now, try commenting out the lines:
apply plugin: 'io.fabric'
and
apply plugin: 'com.google.gms.google-services'
which are found near the beginning and end of the file, respectively.
Let me know if you still have any issues after trying that.