Use latest Docker in CI

Force a Docker upgrade for all Linux runners.

Fixes: #1294

.travis.yml

@@ -1,5 +1,4 @@
 dist: bionic
-sudo: required
 language: bash
 
 services:
@@ -31,8 +30,14 @@ matrix:
         - PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig:$PKG_CONFIG_PATH"
 
 before_install:
-  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then
-    docker build -f Dockerfile."$BUILD_FLAVOR" -t profanity .;
+  - >
+    if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+      # Ensure that Travis uses the latest version of Docker.
+      curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+      sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+      sudo apt-get update
+      sudo apt-get -y -o Dpkg::Options::="--force-confnew" install docker-ce
+      docker build -f Dockerfile."$BUILD_FLAVOR" -t profanity .
     fi
 
 script: