Reduce clone depth of official-images on Travis CI

Only the latest version of official-images repo is needed.

Author: PeterDaveHello

.travis.yml

@@ -36,7 +36,7 @@ env:
   - VERSION=2.7 VARIANT=alpine3.6
 
 install:
-  - git clone https://github.com/docker-library/official-images.git ~/official-images
+  - git clone --depth 1 https://github.com/docker-library/official-images.git ~/official-images
 
 before_script:
   - env | sort