Pin Haml to versions before Haml 6

middleman-syntax does not support Haml 6 [[1]]. However, it does not
place any restriction on the version of Haml used, so bundler will
try and install the latest version of Haml. This commit adds a
restriction on the version of Haml in our gemspec. This is a bit of a
hack, as we don't actually require Haml ourselves, but it will make our
tests pass and provide a better experience for our users until
middleman-syntax is fixed.

[1]: middleman/middleman-syntax#80

Author: lfdebrux

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Fixes
+
+- You no longer need to downgrade Haml yourself, `bundle install` will now make sure Haml 6 is not installed (see issue [#318: Error: Filters is not a module](https://github.com/alphagov/tech-docs/gem/issues/318)).
+
 ## 3.3.1
 
 This change solves a potential security issue with HTML snippets. Pages indexed in search results have their entire contents indexed, including any HTML code snippets. These HTML snippets would appear in the search results unsanitised, making it possible to render arbitrary HTML or run arbitrary scripts.

govuk_tech_docs.gemspec

@@ -47,6 +47,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "nokogiri"
   spec.add_dependency "openapi3_parser", "~> 0.9.0"
   spec.add_dependency "redcarpet", "~> 3.5.1"
+  spec.add_dependency "haml", "< 6.0.0"
 
   spec.add_development_dependency "byebug"
   spec.add_development_dependency "capybara", "~> 3.32"