Remove docker cache

.travis.yml

@@ -10,16 +10,6 @@ services:
 notifications:
   email: false
 
-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'
-
-cache:
-  directories:
-    - $HOME/docker
-
 matrix:
   include:
     - os: osx
@@ -37,10 +27,6 @@ matrix:
         - CXX_VERSION=clang++-6.0
         - CC_VERSION=clang-6.0
 
-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
-
 install:
   - ./.ci/install.sh