Fold Travis output

Created by: ollieparanoid

Right now, the travis output is pretty unreadable. It would be nice if we had folding for at least every command in the script section of our travis.yml: https://github.com/postmarketOS/pmbootstrap/blob/2f78dad318d5c28e466abff895f4ade9cf0aaccb/.travis.yml#L11-L16

Bonus points for also folding builds of individual packages in test/check_checksums.py --build (like Alpine does it).