Enforce gofmt in travis

Run gofmt -l . during travis to catch pull requests that have not had
gofmt run on them.

Change-Id: Ibfdf988a535d82721f3c4bc328b189f606f12778

.travis.gofmt.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -n "$(gofmt -l .)" ]; then
+    echo "Go code is not formatted:"
+    gofmt -d .
+    exit 1
+fi

.travis.yml

@@ -15,6 +15,7 @@ before_script:
     - source .travis.fix-fork.sh
 
 script:
+    - ./.travis.gofmt.sh
     - go test ./...
     - mkdir stage
     - cd stage