FIX(build): py2 needs pinning networkx-2.2

[ankostis]

networkx dependency was pinned on Python-2.x because travis builds on PY2 were failing because of this err:

312NetworkX 2.3+ requires Python 3.5 or later (2.7 detected).
313             
314For Python 2.7, please install version 2.2 using:
315
316$ pip install 'networkx==2.2'

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[codecov-io]

Codecov Report

Merging #16 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #16   +/-   ##
=======================================
  Coverage   77.87%   77.87%           
=======================================
  Files           5        5           
  Lines         348      348           
=======================================
  Hits          271      271           
  Misses         77       77

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