Revert "Try adding Docker caching for Travis CI"

This reverts commit 1990efdba7.

.travis.yml

@@ -1,21 +1,7 @@
 ---
-# 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