bug 1159960 - Update docker tester image. r=lightsofapollo

testing/docker/tester/Dockerfile

@@ -18,6 +18,7 @@ RUN chmod u+x /usr/local/bin/linux64-minidump_stackwalk
 RUN apt-get install -y python-pip && pip install virtualenv;
 RUN mkdir Documents; mkdir Pictures; mkdir Music; mkdir Videos; mkdir artifacts
 RUN npm install -g taskcluster-vcs@2.3.4
+RUN npm install -g taskcluster-npm-cache@1.1.12
 RUN rm -Rf .cache && mkdir -p .cache
 ENV PATH $PATH:/home/worker/bin
 

testing/docker/tester/VERSION

@@ -1 +1 @@
-0.2.15
+0.3.1