Lower required matplotlib version on 2.6 test

tools/travis_before_install.sh

@@ -33,8 +33,10 @@ retry () {
 echo "cython>=0.21" >> requirements.txt
 
 # require networkx 1.9.1 on 2.6, as 2.6 support was dropped in 1.10
+# require matplotlib 1.4.3 on 2.6, as 2.6 support was dropped in 1.5
 if [[ $TRAVIS_PYTHON_VERSION == 2.6* ]]; then
     sed -i 's/networkx.*/networkx==1.9.1/g' requirements.txt
+    sed -i 's/matplotlib.*/matplotlib==1.4.3/g' requirements.txt
 fi
 
 # test minimum requirements on 2.7