macをsierraにアップデートしたら、↓こんな感じのエラーが出ました。 node_modulesを削除して npm install をしなおしたら解決 :)

> starter@0.0.0 start /Users/user_name/Documents/app/training_front_01
> gulp

[20:44:14] Requiring external module babel-register
    throw new Error(errors.unsupportedEnvironment());

Error: Node Sass does not yet support your current environment: OS X 64-bit with Unsupported runtime (51)
For more information on which environments are supported please see:
    at Object.<anonymous> (/Users/user_name/Documents/app/training_front_01/node_modules/node-sass/lib/index.js:13:11)
    at Module._compile (module.js:571:32)
    at Module._extensions..js (module.js:580:10)
    at Object.require.extensions.(anonymous function) [as .js] (/Users/user_name/Documents/app/training_front_01/node_modules/babel-register/lib/node.js:152:7)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/user_name/Documents/app/training_front_01/node_modules/gulp-sass/index.js:183:21)

npm ERR! Darwin 16.6.0
npm ERR! argv "/usr/local/Cellar/node/7.8.0/bin/node" "/usr/local/bin/npm" "start"
npm ERR! node v7.8.0
npm ERR! npm  v4.2.0
npm ERR! errno 1
npm ERR! starter@0.0.0 start: `gulp`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the starter@0.0.0 start script 'gulp'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the starter package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     gulp
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs starter
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls starter
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/user_name/.npm/_logs/2017-05-27T11_44_16_563Z-debug.log

参照:Node Sass does not yet support your current environment (macOS 10.12.1, Node 7.0.0) · Issue #1764 · sass/node-sass · GitHub