update PHPUnit to 6.3

 - symfony/yaml removed (installed version was v3.3.9)

 - sebastian/recursion-context updated from 2.0.0 to 3.0.0
   See changes: sebastianbergmann/recursion-context@2.0.0...3.0.0
   Release notes: https://github.com/sebastianbergmann/recursion-context/releases/tag/3.0.0

 - sebastian/object-reflector installed in version 1.1.1
   Release notes: https://github.com/sebastianbergmann/object-reflector/releases/tag/1.1.1

 - sebastian/object-enumerator updated from 2.0.1 to 3.0.3
   See changes: sebastianbergmann/object-enumerator@2.0.1...3.0.3
   Release notes: https://github.com/sebastianbergmann/object-enumerator/releases/tag/3.0.3

 - sebastian/global-state updated from 1.1.1 to 2.0.0
   See changes: sebastianbergmann/global-state@1.1.1...2.0.0
   Release notes: https://github.com/sebastianbergmann/global-state/releases/tag/2.0.0

 - sebastian/exporter updated from 2.0.0 to 3.1.0
   See changes: sebastianbergmann/exporter@2.0.0...3.1.0
   Release notes: https://github.com/sebastianbergmann/exporter/releases/tag/3.1.0

 - sebastian/environment updated from 2.0.0 to 3.1.0
   See changes: sebastianbergmann/environment@2.0.0...3.1.0
   Release notes: https://github.com/sebastianbergmann/environment/releases/tag/3.1.0

 - sebastian/diff updated from 1.4.3 to 2.0.1
   See changes: sebastianbergmann/diff@1.4.3...2.0.1
   Release notes: https://github.com/sebastianbergmann/diff/releases/tag/2.0.1

 - sebastian/comparator updated from 1.2.4 to 2.0.2
   See changes: sebastianbergmann/comparator@1.2.4...2.0.2
   Release notes: https://github.com/sebastianbergmann/comparator/releases/tag/2.0.2

 - phpunit/phpunit-mock-objects updated from 3.4.4 to 4.0.4
   See changes: sebastianbergmann/phpunit-mock-objects@3.4.4...4.0.4
   Release notes: https://github.com/sebastianbergmann/phpunit-mock-objects/releases/tag/4.0.4

 - theseer/tokenizer installed in version 1.1.0
   Release notes: https://github.com/theseer/tokenizer/releases/tag/1.1.0

 - phpunit/php-code-coverage updated from 4.0.8 to 5.2.2
   See changes: sebastianbergmann/php-code-coverage@4.0.8...5.2.2
   Release notes: https://github.com/sebastianbergmann/php-code-coverage/releases/tag/5.2.2

 - phar-io/version installed in version 1.0.1
   Release notes: https://github.com/phar-io/version/releases/tag/1.0.1

 - phar-io/manifest installed in version 1.0.1
   Release notes: https://github.com/phar-io/manifest/releases/tag/1.0.1

 - phpunit/phpunit updated from 5.7.21 to 6.3.0
   See changes: sebastianbergmann/phpunit@5.7.21...6.3.0
   Release notes: https://github.com/sebastianbergmann/phpunit/releases/tag/6.3.0

composer.json

@@ -20,7 +20,7 @@
         "php": "^7.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "~5.7"
+        "phpunit/phpunit": "^6.3"
     },
     "autoload": {
         "psr-0": { "org\\bovigo\\vfs\\": "src/main/php" }