Update ruby-haml to 5.0.3.

## 5.0.3

Released on September 7, 2017
([diff](https://github.com/haml/haml/compare/v5.0.2...v5.0.3)).

* Use `String#dump` instead of `String#inspect` to generate string
  literal. (Takashi Kokubun)
* Fix Erubi superclass mismatch
  error. [#952](https://github.com/haml/haml/pull/952) (thanks [Robin
  Daugherty](https://github.com/RobinDaugherty))

## 5.0.2

Released on August 1, 2017
([diff](https://github.com/haml/haml/compare/v5.0.1...v5.0.2)).

* Let `haml -c` fail if generated Ruby code is syntax
  error. [#880](https://github.com/haml/haml/issues/880) (Takashi Kokubun)
* Fix `NoMethodError` bug caused with Sprockets 3 and :sass
  filter. [#930](https://github.com/haml/haml/pull/930) (thanks [Gonzalez
  Maximiliano](https://github.com/emaxi))
* Fix `list_of` helper with multi-line
  content. [#933](https://github.com/haml/haml/pull/933) (thanks [Benoit
  Larroque](https://github.com/zetaben))
* Optimize rendering performance by changing timing to fix
  textareas. [#941](https://github.com/haml/haml/pull/941) (Takashi Kokubun)
* Fix `TypeError` with empty :ruby
  filter. [#942](https://github.com/haml/haml/pull/942) (Takashi Kokubun)
* Fix inconsistent attribute sort order. (Takashi Kokubun)

(taca)