enable docker

Author: Jorge Aparicio

.travis.yml

@@ -62,7 +62,8 @@ matrix:
             - qemu-user-static
     - env: TARGET=powerpc64le-unknown-linux-gnu
       os: linux
-      services: docker
+      services:
+        - docker
       sudo: required
     - env: TARGET=x86_64-apple-darwin
       os: osx