Update PHPCSFixes to conform with PHP71

Author: bensquire

.php_cs.dist

@@ -10,6 +10,7 @@ return PhpCsFixer\Config::create()
     ->setRiskyAllowed(true)
     ->setRules([
         '@PSR2' => true,
+        '@PHP71Migration' => true,
         'psr4' => true,
         'array_syntax' => ['syntax' => 'short'],
         'no_short_echo_tag' => true,

src/PHPImageOptim/Tools/ToolsInterface.php

@@ -3,9 +3,9 @@
 
 interface ToolsInterface
 {
-    const OPTIMISATION_LEVEL_BASIC = 1;
-    const OPTIMISATION_LEVEL_STANDARD = 2;
-    const OPTIMISATION_LEVEL_EXTREME = 3;
+    public const OPTIMISATION_LEVEL_BASIC = 1;
+    public const OPTIMISATION_LEVEL_STANDARD = 2;
+    public const OPTIMISATION_LEVEL_EXTREME = 3;
 
     public function optimise(): ToolsInterface;