More perceptualdiff testing tweaks.

.travis.yml

@@ -4,3 +4,5 @@ node_js:
 before_script:
   - "npm install -g grunt-cli"
   - "npm install"
+  - "sudo apt-get update --quiet=2"
+  - "sudo apt-get --quiet=2 install perceptualdiff"

Gruntfile.js

@@ -63,6 +63,7 @@ module.exports = function (grunt) {
         command: './run.sh',
         options: {
           stdout: true,
+          failOnError: true,
           execOptions: {
             cwd: 'spec/viz'
           }