Change the matplotlib dependency

Author: BichengYing

.travis.yml

@@ -29,6 +29,7 @@ addons:
 before_install:
     - ./scripts/install_mpi.sh $MPI
     - pip$PY install torch==1.4.0 tensorflow==2.1.0
+    - pip$PY install # For the plot in doc
     - python$PY --version
     - python$PY -c "import torch; print(torch.__version__)"
     - python$PY -c "import tensorflow as tf; print(tf.__version__)"

requirements.txt

@@ -1,6 +1,5 @@
 cloudpickle
 networkx>=2.0
-matplotlib
 psutil
 pytest>=5.0
 six