Remove 1.11 (now EOL) and promote 1.13 as "latest"

Author: tianon

.appveyor.yml

@@ -7,8 +7,6 @@ environment:
       variant: windowsservercore-ltsc2016
     - version: 1.12
       variant: windowsservercore-ltsc2016
-    - version: 1.11
-      variant: windowsservercore-ltsc2016
 
 install:
   - ps: |

.travis.yml

@@ -21,17 +21,6 @@ matrix:
       env: VERSION=1.12 VARIANT=alpine3.10
     - os: linux
       env: VERSION=1.12 VARIANT=alpine3.9
-    - os: windows
-      dist: 1803-containers
-      env: VERSION=1.11 VARIANT=windows/windowsservercore-1803
-    - os: linux
-      env: VERSION=1.11 VARIANT=buster
-    - os: linux
-      env: VERSION=1.11 VARIANT=stretch
-    - os: linux
-      env: VERSION=1.11 VARIANT=alpine3.10
-    - os: linux
-      env: VERSION=1.11 VARIANT=alpine3.9
 
 install:
   - git clone https://github.com/docker-library/official-images.git ~/official-images