gulp watchで"Error: Cannot find module 'bower'"というエラーが出たので．

# gulp watchで"Error: Cannot find module 'bower'"というエラーが出たので，修正．

$ gulp watch
> module.js:338
> throw err;
>　　　　^
> Error: Cannot find module 'bower'
> at Function.Module._resolveFilename (module.js:336:15)
> at Function.Module._load (module.js:286:25)
> at Module.require (module.js:365:17)
> at require (module.js:384:17)
> at Object. (/PATH/TO/app_v5/gulpfile.js:3:13)
> at Module._compile (module.js:430:26)
> at Object.Module._extensions..js (module.js:448:10)
> at Module.load (module.js:355:32)
> at Function.Module._load (module.js:310:12)
> at Module.require (module.js:365:17)
> at require (module.js:384:17)

$ npm install bower --save-dev
> ... (略)

$git diff package.json
++ "bower": "^1.4.1",

# success
$ gulp watch
> ... (略)