Bump RuboCop to 0.28.0

`Style/EmptyElse` cop offenses:
- https://github.com/khwon/loco/blob/6554180ae4bbcb6cbd0dd57a1eedfb8a742ef162/termapp/processors/read_board_menu.rb#L62
- https://github.com/khwon/loco/blob/6554180ae4bbcb6cbd0dd57a1eedfb8a742ef162/termapp/processors/read_board_menu.rb#L295

If you want to remove any of these, I'll correct them.

I added the `--display-cop-names` option to RuboCop task by default. It shows cop names to terminal on `rake rubocop`.

Before:
```
termapp/processors/read_board_menu.rb:62:13: C: Redundant empty else-clause.
            else
            ^^^^
termapp/processors/read_board_menu.rb:295:7: C: Redundant empty else-clause.
      else nil
      ^^^^
```

After:
```
termapp/processors/read_board_menu.rb:62:13: C: Style/EmptyElse: Redundant empty else-clause.
            else
            ^^^^
termapp/processors/read_board_menu.rb:295:7: C: Style/EmptyElse: Redundant empty else-clause.
      else nil
      ^^^^
```

Also you can use `rubocop -D` on your shell directly.