Add missing dev dependency js-beautify.

[vanam]

I run command npm run watch-pug and got following error

> @coreui/coreui-free-bootstrap-admin-template@2.1.0 watch-pug /home/martin/Programming/Frontend/coreui-free-bootstrap-admin-template
> nodemon -e pug -x "npm run pug"

[nodemon] 1.17.4
[nodemon] to restart at any time, enter `rs`
[nodemon] watching: *.*
[nodemon] starting `npm run pug`

> @coreui/coreui-free-bootstrap-admin-template@2.1.0 pug /home/martin/Programming/Frontend/coreui-free-bootstrap-admin-template
> node build/pug.js

module.js:559
    throw err;
    ^

Error: Cannot find module 'js-beautify'
    at Function.Module._resolveFilename (module.js:557:15)
    at Function.Module._load (module.js:484:25)
    at Module.require (module.js:606:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/home/martin/Programming/Frontend/coreui-free-bootstrap-admin-template/build/pug.js:14:20)
    at Module._compile (module.js:662:30)
    at Object.Module._extensions..js (module.js:673:10)
    at Module.load (module.js:575:32)
    at tryModuleLoad (module.js:515:12)
    at Function.Module._load (module.js:507:3)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @coreui/coreui-free-bootstrap-admin-template@2.1.0 pug: `node build/pug.js`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @coreui/coreui-free-bootstrap-admin-template@2.1.0 pug script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/martin/.npm/_logs/2018-07-05T08_21_38_623Z-debug.log
[nodemon] app crashed - waiting for file changes before starting...

Therefore, I added missing dependency.