Use the CI env variable

Travis exports this variable to all builds. We don't need out own.
A number of other CI systems use this variable too.

.travis.yml

@@ -11,7 +11,7 @@ go:
   - tip
 
 env:
-  - TRAVIS_RUN=true DOCKER_VERSION=1.9.1
+  - DOCKER_VERSION=1.9.1
 
 matrix:
   allow_failures:

testutil/wait.go

@@ -11,7 +11,7 @@ import (
 const (
 	// TravisRunEnv is an environment variable that is set if being run by
 	// Travis.
-	TravisRunEnv = "TRAVIS_RUN"
+	TravisRunEnv = "CI"
 )
 
 type testFn func() (bool, error)