Docker environment breaking (commaai#369)

* Fixes matplotlib dependency

* Use version 2.1.2

Author: energee

Dockerfile.openpilot

@@ -2,7 +2,7 @@ FROM ubuntu:16.04
 ENV PYTHONUNBUFFERED 1
 
 RUN apt-get update && apt-get install -y build-essential clang vim screen wget bzip2 git libglib2.0-0 python-pip capnproto libcapnp-dev libzmq5-dev libffi-dev libusb-1.0-0
-RUN pip install numpy==1.11.2 scipy==0.18.1 matplotlib
+RUN pip install numpy==1.11.2 scipy==0.18.1 matplotlib==2.1.2
 
 COPY requirements_openpilot.txt /tmp/
 RUN pip install -r /tmp/requirements_openpilot.txt