Fix issue supportsSchemaTransactions() on null

got this issue while fixing existing issue fix this issue 
Call to a member function supportsSchemaTransactions() on null
by adding Grammar.php and editing Connection.php

src/Schema/Grammar.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace duxet\RethinkDB\Schema;
+
+use Illuminate\Database\Schema\Grammars\Grammar as BaseGrammar;
+
+/**
+ * Class Grammar
+ *
+ * @package Moloquent\Schema
+ */
+class Grammar extends BaseGrammar {
+}