Adjust PHPUnit 5 check to be consistent with others

This is consistent with how other versions are checked, and also reduces
esoteric bugs when PHPUnit_TextUI_ResultPrinter exists, even though the
PHPUnit version is newer.

src/Printer.php

@@ -5,14 +5,20 @@
 use PHPUnit\Runner\Version;
 use PHPUnit_TextUI_ResultPrinter;
 
-if (class_exists(PHPUnit_TextUI_ResultPrinter::class)) {
+$low  = version_compare(Version::series(), '5.0', '>=');
+$high = version_compare(Version::series(), '5.99.99', '<=');
+
+if ($low && $high) {
     class Printer extends ResultPrinter5
     {
         use PrinterTrait;
     }
 }
 
-if (version_compare(Version::series(), '6.99.99', '<=')) {
+$low  = version_compare(Version::series(), '6.0', '>=');
+$high = version_compare(Version::series(), '6.99.99', '<=');
+
+if ($low && $high) {
     class Printer extends ResultPrinter6
     {
         use PrinterTrait;