Fix failing migrations

Author: DaneEveritt

database/migrations/2016_02_27_163411_add_tasks_table.php

@@ -23,8 +23,8 @@ public function up()
             $table->string('day_of_month')->default('*');
             $table->string('hour')->default('*');
             $table->string('minute')->default('*');
-            $table->timestamp('last_run');
-            $table->timestamp('next_run');
+            $table->timestamp('last_run')->nullable();
+            $table->timestamp('next_run')->nullable();
             $table->timestamps();
         });
     }