Manually invoke apt-get update and install instead of using addon

This allows us to use travis_retry on these operations.

Author: codablock

.travis.yml

@@ -8,19 +8,10 @@ dist: trusty
 os: linux
 language: generic
 
-addons:
-  apt:
-    packages:
-      # Use more recent docker version
-      - docker-ce
-      # Can be removed if Travis ever upgrades to Bionic
-      - realpath
-
 services:
   - docker
 
 cache:
-  apt: true
   ccache: true
   directories:
     - $HOME/cache
@@ -122,6 +113,10 @@ before_cache:
   # Save builder image
   - docker save dash-builder-$BUILD_TARGET-$TRAVIS_JOB_NUMBER $(docker history -q dash-builder-$BUILD_TARGET-$TRAVIS_JOB_NUMBER | grep -v \<missing\>) | gzip -2 > $HOME/cache/docker/dash-builder-$BUILD_TARGET.tar.gz
 
+before_install:
+  - travis_retry travis_apt_get_update
+  - travis_retry sudo apt-get -yq --no-install-suggests --no-install-recommends install docker-ce realpath
+
 install:
     # Our scripts try to be Travis agnostic
     - export PULL_REQUEST="$TRAVIS_PULL_REQUEST"