Install matplotlib with pip rather than system-wide

Dockerfile

@@ -24,7 +24,6 @@ RUN apt-get install --install-recommends --assume-yes \
         python2.7 \
         python2.7-dev \
         python-glue \
-        python-matplotlib \
         python-pip \
         python-voeventlib \
         procps \

requirements.txt

@@ -16,6 +16,7 @@ ipython==5.5.0
 ligo-lvalert==1.5.4
 ligo-lvalert-overseer==0.1.2
 lxml==4.2.0
+matplotlib==2.0.0
 mock==2.0.0
 mysql-python==1.2.5
 packaging==17.1