Updating to support L7

Accepting null as the first param of the convertToLocal method. (jame…

…smills#19)

* Accepting null

* Docs

Updating changelog for new release

Updating changelog for new release

Merge branch 'master' of https://github.com/jamesmills/laravel-timezone

Updated changlog for new release

Added config option for overwriting database value

Updating CHANGLOG

Update the required version of Carbon

Updating for Laravel 5.7 & 5.8