removed the Worker model and related services and payloads

also updated the schema from when the Workers table was removed.
Finish the build when a job fails, or if the only remaining jobs allow failures
Config should raise an error if both file and env configs are available

We use file configs for some apps and ENV config for the others. When
both configs are available (for example because someone accidentally set
an ENV config where file config is already set), Travis.config will
silently prefer ENV over file config.

This commit raises an error in such situation