Test using newer versions of PHPUnit

Author: colinodell

.gitignore

@@ -1,2 +1,4 @@
+.phpunit.result.cache
 composer.lock
+phpunit.xml
 vendor

composer.json

@@ -26,7 +26,7 @@
         "php": "^7.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0"
+        "phpunit/phpunit": "^7.5 || ^8.5 || ^9.3"
     },
     "autoload": {
         "psr-4": {

phpunit.xml.dist

@@ -1,5 +1,14 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit colors="true">
+<phpunit bootstrap="vendor/autoload.php"
+         backupGlobals="false"
+         backupStaticAttributes="false"
+         colors="true"
+         verbose="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false">
     <testsuites>
         <testsuite name="Dot Access Data Test Suite">
             <directory>./tests</directory>