Upgrade system for Python 3.5 for decision task

NetworkX upgrade has no more support for Python 3.4

Fixes #2045
X-DeepSpeech: NOBUILD

.taskcluster.yml

@@ -41,7 +41,7 @@ tasks:
 
     payload:
       maxRunTime: 600
-      image: "ubuntu:14.04"
+      image: "ubuntu:16.04"
 
       features:
         taskclusterProxy: true
@@ -56,7 +56,7 @@ tasks:
         - >
           echo "deb http://archive.ubuntu.com/ubuntu/ trusty-updates main" > /etc/apt/sources.list.d/trusty-updates.list &&
           sed -ri -e "s|archive.ubuntu.com|${TASKCLUSTER_WORKER_GROUP}.ec2.archive.ubuntu.com|g" -e "s|security.ubuntu.com|${TASKCLUSTER_WORKER_GROUP}.ec2.archive.ubuntu.com|g" /etc/apt/sources.list &&
-          apt-get -qq update && apt-get -qq -y install git python3-pip curl &&
+          apt-get -qq update && apt-get -qq -y install git python3-pip curl sudo &&
           adduser --system --home /home/build-user build-user &&
           cd /home/build-user/ &&
           echo -e "#!/bin/bash\nset -xe\nenv && id && mkdir ~/DeepSpeech/ && git clone --quiet {{event.head.repo.url}} ~/DeepSpeech/ds/ && cd ~/DeepSpeech/ds && git checkout --quiet {{event.head.sha}}" > /tmp/clone.sh && chmod +x /tmp/clone.sh &&

tc-schedule.sh

@@ -6,6 +6,8 @@ curdir=$(dirname "$0")
 
 pip3 install --quiet --user --upgrade pip
 
+export PATH=$HOME/.local/bin/:$PATH
+
 curl -L --silent https://raw.githubusercontent.com/lissyx/taskcluster-github-decision/${TC_DECISION_SHA}/requirements.txt | pip3 install --quiet --user --upgrade -r /dev/stdin
 curl -L --silent https://raw.githubusercontent.com/lissyx/taskcluster-github-decision/${TC_DECISION_SHA}/tc-decision.py > ${curdir}/tc-decision.py