Don't use a bad 2.10 version
"Require" sources in JobTests to have a mock buffer.

This is just adding an error message to inform users writing JobTests that they need to
provide buffers for every source used in the job.
Merge pull request #759 from daisyzhou/develop

"Require" sources in JobTests to have a mock buffer.
Merge pull request #756 from twitter/fix210ver

Don't use a bad 2.10 version