Detect execution inside VSTS/TFS build agent

Author: jessehouwing

lib/is-ci.js

@@ -12,6 +12,7 @@ var ciEnvs = [
   'PHPCI',
   'GOCD_SERVER_HOST',
   'BUILDKITE',
+  'TF_BUILD'
 ];
 
 module.exports = !!Object.keys(process.env).filter(function (env) {