Change migration to bigint (#22)

fixes issue #21

migrations/2017_11_24_094130_create_stripes_table.php

@@ -19,7 +19,7 @@ public function up()
             $table->string('account_id')->nullable();
             $table->string('customer_id')->nullable();
 
-            $table->integer('user_id')->unsigned()->index();
+            $table->bigInteger('user_id')->unsigned()->index();
             $table->foreign('user_id')->references('id')->on('users');
 
             $table->timestamps();