Move to wpull 0.29.
Capture wpull --version for each job.

This eliminates false user-agent strings when wpull is upgraded without
restarting the pipeline.
Pipeline: eliminate wpull version check.

My reading of seesaw.util.test_executable indicates that a >= check
could be done, but not directly.  Easier solution: just don't bother.
Most people will be running ArchiveBot pipelines by installing
dependencies via pip, and that should get things sorted out on most
machines.
Eliminate unused variables.
Bump pipeline version.