Adds the ability to define a getMixPanelKey method on the user class …

[Arkitecht]

Adds the ability to define a getMixPanelKey method on the user class to use for the identifier

• Addresses Issue 54
• Adds tests for default identifier and alternative identifier
• Updates composer to add laravel/ui which is now needed for Auth routes
• Adds config in Fixtures to load the Laravel\UI\UiServiceProvider for Auth routes

[Arkitecht]

Unfortunately Travis is failing due to composer memory issues

PHP Fatal error: Allowed memory size of 1610612736 bytes exhausted (tried to allocate 4096 bytes) in phar:///home/travis/.phpenv/versions/7.3.22/bin/composer/src/Composer/DependencyResolver/Solver.php on line 223

[mikebronner]

@Arkitecht thanks for this PR. I will review it this weekend. :)

[juancho48]

COMPOSER_MEMORY_LIMIT=-1 might help to run Travis

[Arkitecht]

Hey @mikebronner - wondering if you had a chance to review this PR. Please let me know if there is anything I can do to help get it in.