Add a guard to fail fast in phar:build when phar.readonly is on.

RoboFile.php

@@ -73,6 +73,8 @@ public function generateTask($className, $wrapperClassName = "")
      */
     public function release($opts = ['beta' => false])
     {
+        $this->checkPharReadonly();
+
         $version = \Robo\Robo::VERSION;
         $stable = !$opts['beta'];
         if ($stable) {
@@ -332,6 +334,8 @@ public function publish()
      */
     public function pharBuild()
     {
+        $this->checkPharReadonly();
+
         // Create a collection builder to hold the temporary
         // directory until the pack phar task runs.
         $collection = $this->collectionBuilder();
@@ -405,6 +409,13 @@ public function pharBuild()
             ->run();
     }
 
+    protected function checkPharReadonly()
+    {
+        if (ini_get('phar.readonly')) {
+            throw new \Exception('Must set "phar.readonly = Off" in php.ini to build phars.');
+        }
+    }
+
     /**
      * The phar:build command removes the project requirements from the
      * 'require-dev' section that are not in the 'suggest' section.