Update phpunit.xml

From deprecated schema. And add .phpunit.cache to .gitignore.

.gitignore

@@ -2,3 +2,4 @@ composer.lock
 vendor
 .php-cs-fixer.cache
 .phpunit.result.cache
+.phpunit.cache

phpunit.xml

@@ -1,16 +1,13 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
-         bootstrap="vendor/autoload.php"
-         colors="true">
-    <testsuites>
-        <testsuite name="Test Suite">
-            <directory suffix="Test.php">./tests</directory>
-        </testsuite>
-    </testsuites>
-    <coverage processUncoveredFiles="true">
-        <include>
-            <directory suffix=".php">./src</directory>
-        </include>
-    </coverage>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd" bootstrap="vendor/autoload.php" colors="true" cacheDirectory=".phpunit.cache">
+  <testsuites>
+    <testsuite name="Test Suite">
+      <directory suffix="Test.php">./tests</directory>
+    </testsuite>
+  </testsuites>
+  <source>
+    <include>
+      <directory suffix=".php">./src</directory>
+    </include>
+  </source>
 </phpunit>