Make compatible with Laravel 5.7

[rodrigopedra]

Updates composer.json and adds new test environments to .travis.yml to make compatible with Laravel 5.7

[rodrigopedra]

I had to tweak the hhvm to keep the tests running on it.

1. Hold on HHVM version 3.18

   It was the last version to fully support trusty, and to be largely compatible with PHP 5.6, which is the version Laravel 5.4 officially supports.

   https://hhvm.com/blog/2017/02/15/hhvm-3-18.html

2. Use phpunit 5.7 when running HHVM tests

   phpunit version 6 and up are not compatible with HHVM:

   Unable to build with travis and hhvm after upgrading to phpunit 6.x sebastianbergmann/phpunit#2581 (comment)

   I found the workaround to use a different phpunit version in this Reddit thread:

   https://www.reddit.com/r/HHVM/comments/95m72p/testing_on_travisci/

[rodrigopedra]

ping @srmklive

[rodrigopedra]

Thanks!