Laravel 5.8 support

[JaZo]

Description

I've added support for Laravel 5.8.

Motivation and context

Laravel 5.8 is out and about! I had to drop PHP <7.1 and Laravel <5.5 support in order to make it compatible. But since these are both EOL, I think it is appropriate to do so!

closes #49

How has this been tested?

Tested with existing unit tests.

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.

• 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 added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[bbrala]

Think older the older testbench/phpunit dependency is still needed for Laravel 5.5.