chore(ci): use the stable Docker repository (#15)

.travis.yml

@@ -17,7 +17,7 @@ install:
   - sudo apt-get -y update
   - sudo apt-get -y install apt-transport-https ca-certificates curl software-properties-common
   - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-  - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) edge"
+  - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
   - sudo apt-get update
   - sudo apt-get -y install docker-ce
   - go get github.com/autonomy/conform