Bump up taskflow to its newest version

Taskflow 0.4 was just released so update the bounds
to be the version that is most useful for the juno
release.

Release notes @

https://etherpad.openstack.org/p/TaskFlow-0.4

Change-Id: Ib261d3d3defdb0fa396b0cef16b322bcce00c64d
Merge "Bump up taskflow to its newest version"