Switch to using the full GCE image by default, and just use the conta…

…iners for the shorter jobs as recommended in OpenLightingProject#1317

This should give us the fastest overall run time.

.travis.yml

@@ -1,5 +1,7 @@
 language: cpp
-sudo: false
+# Default to the fully visualised "sudo" GCE environments, as they are faster for longer running jobs. We don't actually need sudo
+sudo: required
+# Use the latest Travis images since they are more up to date than the stable release.
 group: edge
 script:
  - "bash -ex .travis-ci.sh"
@@ -108,6 +110,8 @@ matrix:
            - ubuntu-toolchain-r-test
     - os: linux
       dist: trusty
+      # Short duration job, use the container/without sudo image as it boots faster
+      sudo: false
       env: TASK='doxygen'
       addons:
         apt:
@@ -119,26 +123,34 @@ matrix:
            - ubuntu-toolchain-r-test
     - os: linux
       dist: trusty
+      # Short duration job, use the container/without sudo image as it boots faster
+      sudo: false
       env: TASK='lint'
       addons:
         apt:
           packages:
            - *core_build
     - os: linux
       dist: trusty
+      # Short duration job, use the container/without sudo image as it boots faster
+      sudo: false
       env: TASK='check-licences'
       addons:
         apt:
           packages:
            - *core_build
     - os: linux
       dist: trusty
+      # Short duration job, use the container/without sudo image as it boots faster
+      sudo: false
       env: TASK='jshint'
       addons:
         apt:
           packages:
     - os: linux
       dist: trusty
+      # Short duration job, use the container/without sudo image as it boots faster
+      sudo: false
       env: TASK='flake8'
       addons:
         apt: