Update gulp-sass version

[elrumordelaluz]

To avoid undesirable errors when build like:

Error: `libsass` bindings not found. Try reinstalling `node-sass`?
    at getBinding (/Applications/MAMP/htdocs/wordpress/react/wp-content/themes/Picard/node_modules/gulp-sass/node_modules/node-sass/lib/index.js:22:11)
    at Object.<anonymous> (/Applications/MAMP/htdocs/wordpress/react/wp-content/themes/Picard/node_modules/gulp-sass/node_modules/node-sass/lib/index.js:188:23)
    at Module._compile (module.js:435:26)
    at Object.Module._extensions..js (module.js:442:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at Object.<anonymous> (/Applications/MAMP/htdocs/wordpress/react/wp-content/themes/Picard/node_modules/gulp-sass/index.js:3:17)
    at Module._compile (module.js:435:26)

Installing ^2 versions solve this.