travis: ignore apt update errors.
travis: ignore apt update errors.

The `apt-get update` commands have already caused some build flakes. We
can ignore these failures. We might get some old versions of packages
sometimes, but that's okay. If a PPA repo fetch fails we'll get an error
that the package doesn't exist, anyway.

GoogleCloudPlatform/cloud-vision,csilzen/whatdoyousee,csilzen/whatdoyousee,GoogleCloudPlatform/cloud-vision,GoogleCloudPlatform/cloud-vision,csilzen/whatdoyousee,csilzen/whatdoyousee,KaranToor/MA450,KaranToor/MA450,KaranToor/MA450,KaranToor/MA450,csilzen/whatdoyousee,csilzen/whatdoyousee,GoogleCloudPlatform/cloud-vision,KaranToor/MA450,csilzen/whatdoyousee,KaranToor/MA450,GoogleCloudPlatform/cloud-vision,KaranToor/MA450,csilzen/whatdoyousee,KaranToor/MA450,csilzen/whatdoyousee