Address networkx's API change, which breaks OnionPerf

Networkx in version 2.4 deprecated the node attribute of DiGraph and suggests using nodes instead. This breaks OnionPerf but luckily, it's an easy fix.

Here's the error I'm getting on master:

Traceback (most recent call last):
  File "/home/phw/rcs/onionperf/venv/bin/onionperf", line 4, in <module>
    __import__('pkg_resources').run_script('OnionPerf==0.2rc0', 'onionperf')
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/pkg_resources/__init__.py", line 666, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/pkg_resources/__init__.py", line 1453, in run_script
    exec(script_code, namespace, namespace)
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/EGG-INFO/scripts/onionperf", line 529, in <module>
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/EGG-INFO/scripts/onionperf", line 350, in main
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/EGG-INFO/scripts/onionperf", line 401, in measure
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/onionperf/measurement.py", line 239, in run
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/onionperf/measurement.py", line 315, in __start_tgen_client
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/onionperf/measurement.py", line 341, in __start_tgen
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/onionperf/model.py", line 66, in __init__
  File "/home/phw/rcs/onionperf/venv/lib/python3.7/site-packages/OnionPerf-0.2rc0-py3.7.egg/onionperf/model.py", line 74, in generate
AttributeError: 'DiGraph' object has no attribute 'node'