Fix timestamp field

Author: jyhsu2000

database/migrations/2015_06_25_142416_create_announcements_table.php

@@ -16,8 +16,8 @@ public function up()
         Schema::create('announcements', function (Blueprint $table) {
             $table->increments('id');
             $table->string('title', 100);
-            $table->timestamp('start_time');
-            $table->timestamp('end_time');
+            $table->timestamp('start_time')->nullable();
+            $table->timestamp('end_time')->nullable();
             $table->text('message');
             $table->timestamps();
         });