CI: install networkx==2.4 on Python 3.5

because version 2.4 is the last version of `networkx`
that supports Python 3.5 [1, 2].

[1] https://pypi.org/project/networkx/2.4/
[2] https://pypi.org/project/networkx/2.5/

.travis.yml

@@ -25,6 +25,7 @@ addons:
 install:
   - pip install --upgrade pip setuptools
   - if test 2 = `python -c 'import sys; print(sys.version_info.major)'`; then pip install networkx==2.2 pyparsing==2.4.7; fi
+  - if test True = `python -c 'import sys; print(sys.version_info.major == 3 and sys.version_info.minor == 5)'`; then pip install networkx==2.4; fi
   - pip install -r requirements.txt
   - curl -L https://github.com/utwente-fmt/sylvan/tarball/v1.0.0 -o sylvan.tar.gz
   - echo '9877fe07a8cfe9889152e29624a4c5b283cb34672ec524ccb3edb313b3057fbf8ef45622a49796fae17aa24e0baea5ccfa18f1bc5923e3c55245ab3e3c1927c8  sylvan.tar.gz' | shasum -a 512 -c -