Remove build folder

[matheusggds]

Remove build folder is a need change, because GitHub doesnt need to have a folder with minificated files.

But, the thing is, that, manifest.json need to be placed inside a folder that have only needed files for the extension, without node_modules, package.json, and other 'useless files', otherwise extension points error when try to upload cus there is to many files, like /nodemodules for examle...

The must need is to manifest.json and background.js stay at /scr/, then, a gulp task copy it to /build/