Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.

composer.jsonを編集します。

"require-dev": {
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "0.9.*",
        "symfony/css-selector": "3.1.*",
        "symfony/dom-crawler": "3.1.*"
    },

updateします。
[vagrant@localhost blog]$ php composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 0 updates, 27 removals
– Removing webmozart/assert (1.4.0)
– Removing theseer/tokenizer (1.1.2)
– Removing sebastian/version (2.0.1)
– Removing sebastian/resource-operations (2.0.1)
– Removing sebastian/recursion-context (3.0.0)
– Removing sebastian/object-reflector (1.1.1)
– Removing sebastian/object-enumerator (3.0.3)
– Removing sebastian/global-state (2.0.0)
– Removing sebastian/exporter (3.1.0)
– Removing sebastian/environment (4.1.0)
– Removing sebastian/diff (3.0.2)
– Removing sebastian/comparator (3.0.2)
– Removing sebastian/code-unit-reverse-lookup (1.0.1)
– Removing phpunit/phpunit (7.5.8)
– Removing phpunit/php-token-stream (3.0.1)
– Removing phpunit/php-timer (2.1.1)
– Removing phpunit/php-text-template (1.2.1)
– Removing phpunit/php-file-iterator (2.0.2)
– Removing phpunit/php-code-coverage (6.1.4)
– Removing phpspec/prophecy (1.8.0)
– Removing phpdocumentor/type-resolver (0.4.0)
– Removing phpdocumentor/reflection-docblock (4.3.0)
– Removing phpdocumentor/reflection-common (1.0.1)
– Removing phar-io/version (2.0.1)
– Removing phar-io/manifest (1.0.3)
– Removing myclabs/deep-copy (1.9.1)
– Removing doctrine/instantiator (1.2.0)
Writing lock file
Generating autoload files
> Illuminate\Foundation\ComposerScripts::postUpdate
> php artisan optimize
Generating optimized class loader
The compiled class file has been removed.
[vagrant@localhost blog]$ php artisan -V
Laravel Framework version 5.3.31

なぜだあああああああああああああああああ