Error: Filters is not a module

[lfdebrux]

Some users have reported seeing this error message after upgrading dependencies:

bundler: failed to load command: middleman (.bundle/bin/middleman)
TypeError: Filters is not a module
  .bundle/gems/middleman-syntax-3.2.0/lib/middleman-syntax/haml_monkey_patch.rb:4:in `<module:Haml>'

It seems to be related to using HAML >= 6, downgrading HAML to ~> 5 works as a workaround.

[lfdebrux]

Fixing this in #328.