chore(dependencies): update convict dependency to v0.6

Older versions of convict depend on an insecure version of the validator
module.
fix(logging): remove spaces from logging op name in the worker

After the switch to mozlog, the first arg to all logging calls is an
"op name" which cannot contain spaces.  This removes one such instance
which was causing (unhandled) errors in the worker process.

Closes #77