Add Laravel 6 support

[christophengelmayer]

Description

Updated dependencies to support Laravel 6.

Changed deprecated package "dimsav/laravel-translatable" to use "astrotomic/laravel-translatable" instead. (As recommended here)
Updated tests and Readme.

Motivation and context

Fixes #33 #31

How has this been tested?

• Run the included tests.
• Successfully installed in an existing Laravel 6 Project

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

Please, please, please, don't send your pull request until all of the boxes are ticked. Once your pull request is created, it will trigger a build on our continuous integration server to make sure your tests and code style pass.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have updated tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

[JaZo]

We've skipped Laravel 6 support and only support Laravel 7+ now.