Try adding Docker caching for Travis CI

.travis.yml

@@ -1,7 +1,21 @@
 ---
+# yamllint disable rule:line-length
 sudo: required
 services:
     - docker
 language: generic
+# Set up Docker caching per: https://github.com/travis-ci/travis-ci/issues/5358#issuecomment-248915326
+before_cache:
+    # Save tagged docker images
+    - >
+      mkdir -p $HOME/docker && docker images -a --filter='dangling=false' --format '{{.Repository}}:{{.Tag}} {{.ID}}'
+      | xargs -n 2 -t sh -c 'test -e $HOME/docker/$1.tar.gz || docker save $0 | gzip -2 > $HOME/docker/$1.tar.gz'
+before_install:
+    # Load cached docker images
+    - if [[ -d $HOME/docker ]]; then ls $HOME/docker/*.tar.gz | xargs -I {file} sh -c "zcat {file} | docker load"; fi
+cache:
+    bundler: true
+    directories:
+        - $HOME/docker
 script: |
     ./run-tests