Framework for non-local branches in ControlFlowVisitor (#1933)
- add support for tracking named 'global' flow paths between different
parts of the IR, useful for tracking data flow over things like
exit/return or calls or state transitions.
python-pip seems to be broken on Ubuntu 16.04. The installation of pip
itself is failing (it seems that it fails to reach PyPI, which may be
specific to the Travis environment, which causes a backtrace).
We also remove Tenjin from the python dependencies, since it is not used
by p4lang/p4c.