[3.6] Make the bootstrap more robust

[StevendeVries]

When using an old version of phpunit like 4.8.28 it does not provide a class called PHPUnit\Framework\TestCase required by bolt/bolt/tests/phpunit/unit/BoltUnitTest.php
The class is available in version 5.7. The dependancy with phpunit/php-code-coverage has also been updated.

Next to the phpunit version there is an issue when running the unittests, it can't seem to find the vendor/autoload.php. I've made the bootstrap a bit more defensive.
For context: We have the bootstrap located in: vendor/bolt/bolt/tests/phpunit/bootstrap.php

[StevendeVries]

This currently breaks being compatible with PHP 5.5.9, however the unittests didn't work properly with PHP 5.5.9 anyway due to the lower phpunit version.

[bobdenotter]

Do you think you could get this fixed for PHP 5.5.9? For Bolt 3, we'd need to keep compatible with that version, i'm afraid.

[StevendeVries]

@bobdenotter if you really want to keep supporting 5.5.9 I would suggest reverting this PR and rewrite BoltUnitTest.php to remove the dependancy on PHPUnit\Framework\TestCase

[bobdenotter]

@StevendeVries We use SemVer, so we cannot drop PHP 5.5.9 support for Bolt 3. Only in a major release can we bump the minimum requirements.

PHPUnit 5.7 should work on PHP 5.5.9 too, right?

[StevendeVries]

Like stated in the ci: phpunit/phpunit 5.7.0 requires php ^5.6 😢

[StevendeVries]

I'll remove the phpunit version increase to keep the bootstrap changes. Making it possible for you to merge.

[bobdenotter]

Thanks, mate! 👍