textproc/ruby-haml: update to 5.1.2

Update ruby-haml to 5.1.2.
pkgsrc change: add "USE_LANGUAGES= # none".

## 5.1.2

Released on August 6, 2019
([diff](https://github.com/haml/haml/compare/v5.1.1...v5.1.2)).

* Fix crash in some environments such as New Relic by unfreezing string literals for ParseNode#inspect. [#1016](https://github.com/haml/haml/pull/1016) (thanks [Jalyna](https://github.com/jalyna))

## 5.1.1

Released on May 25, 2019
([diff](https://github.com/haml/haml/compare/v5.1.0...v5.1.1)).

* Fix NameError bug for that happens on ruby 2.6.1-2.6.3 + haml 5.1.0 + rails 4.2.x + erubi. (Akira Matsuda)

## 5.1.0

Released on May 16, 2019
([diff](https://github.com/haml/haml/compare/v5.0.4...v5.1.0)).

* Rails 6 support [#1008](https://github.com/haml/haml/pull/1008) (thanks [Seb Jacobs](https://github.com/sebjacobs))
* Add `escape_filter_interpolations` option for backwards compatibility with haml 4 defaults [#984](https://github.com/haml/haml/pull/984) (thanks [Will Jordan](https://github.com/wjordan))
* Fix error on empty :javascript and :css filter blocks [#986](https://github.com/haml/haml/pull/986) (thanks [Will Jordan](https://github.com/wjordan))
* Respect changes in Haml::Options.defaults in `Haml::TempleEngine` options (Takashi Kokubun)
* Un-freeze TempleEngine precompiled string literals [#983](https://github.com/haml/haml/pull/983) (thanks [Will Jordan](https://github.com/wjordan))
* Various performance/memory improvements [#965](https://github.com/haml/haml/pull/965), [#966](https://github.com/haml/haml/pull/966), [#963](https://github.com/haml/haml/pull/963) (thanks [Dillon Welch](https://github.com/oniofchaos))
* Enable `frozen_string_literal` magic comment for all .rb files [#967](https://github.com/haml/haml/pull/967) (thanks [Dillon Welch](https://github.com/oniofchaos))

(taca)