Update 2013_04_08_175152_create_role_user_table.php

By adding this unsigned() to migration

You can then have an extra migration with this 

/ Keys
                $table->foreign('user_id')
                      ->references('id')->on('users')
                      ->onDelete('cascade');

                $table->foreign('role_id')
                      ->references('id')->on('roles')
                      ->onDelete('cascade');