Remove the vendor directory in .gitignore as these dependencies are needed for php-imap

plugins/php-imap/vendor/psr/simple-cache/src/InvalidArgumentException.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Psr\SimpleCache;
+
+/**
+ * Exception interface for invalid cache arguments.
+ *
+ * When an invalid argument is passed it must throw an exception which implements
+ * this interface
+ */
+interface InvalidArgumentException extends CacheException
+{
+}