* with-firebase-hosting: update next.js 6, readme about customization
* now next.js 6 is on Babel 7, remove conflicting babel deps
* update Next Server/Cloud Function .babelrc config
* update other deps: Cloud Functions to 1.x.x etc
* rm install-deps script as it is no longer used on deployment (firebase does not upload node_modules)
* make scripts consistent in their wrapping of dirs with \" (escaped double quotes)
* with-firebase-hosting: pin next to "latest" version
* with-firebase-hosting: fix lint errors
* Next.js v5 update, _error & _document bugs fixes
* document clean script
* remove old build artefact
* add babelrc
* reset next.js version to 'latest' from '^5.0.0'
* typo in readme
* format code
* remove _files as they were a fix for a solved, but unreleased problem
