Update slim_lint requirement from ~> 0.27.0 to ~> 0.28.0 (#555)

Updates the requirements on
[slim_lint](https://github.com/sds/slim-lint) to permit the latest
version.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/sds/slim-lint/releases">slim_lint's
releases</a>.</em></p>
<blockquote>
<h2>v0.28.0</h2>
<ul>
<li>Add Tag linter in <a
href="https://redirect.github.com/sds/slim-lint/pull/179">sds/slim-lint#179</a></li>
</ul>
<p><strong>Full Changelog</strong>: <a
href="https://github.com/sds/slim-lint/compare/v0.27.0...v0.28.0">https://github.com/sds/slim-lint/compare/v0.27.0...v0.28.0</a></p>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/sds/slim-lint/blob/main/CHANGELOG.md">slim_lint's
changelog</a>.</em></p>
<blockquote>
<h2>0.28.0</h2>
<ul>
<li>Add <code>Tag</code> linter for restricting tag usage</li>
</ul>
<h2>0.27.0</h2>
<ul>
<li>Add <code>InstanceVariables</code> linter</li>
<li>Add <code>StrictLocalsMissing</code> linter</li>
<li>Fix <code>EmbeddedEngines</code> linter missing indented uses</li>
</ul>
<h2>0.26.0</h2>
<ul>
<li>Revert &quot;Fix <code>ControlSpacingStatement</code> linter
handling of <code>=</code> in some cases&quot;</li>
</ul>
<h2>0.25.0</h2>
<ul>
<li>Drop support for Ruby 2.x</li>
<li>Fix <code>ControlSpacingStatement</code> linter handling of
<code>=</code> in some cases</li>
</ul>
<h2>0.24.0</h2>
<ul>
<li>Require RuboCop 1.x or newer</li>
<li>Silence Parser gem warnings on startup</li>
</ul>
<h2>0.23.0</h2>
<ul>
<li>Add <code>Zwsp</code> to report zero-width space characters</li>
<li>Allow Slim version 5.x</li>
</ul>
<h2>0.22.1</h2>
<ul>
<li>Fix <code>CommentControlStatement</code> to not report
<code>Template Dependency:</code> directives</li>
<li>Fix handling of disabled lines when running across multiple
files</li>
<li>Add <code>Layout/ClosingParenthesisIndentation</code> to list of
ignored RuboCop cops</li>
<li>Add <code>Layout/FirstHashElementIndentation</code> to list of
ignored RuboCop cops</li>
</ul>
<h2>0.22.0</h2>
<ul>
<li>Disable RuboCop <code>Layout/FirstArgumentIndentation</code></li>
<li>Disable RuboCop
<code>Layout/LineEndStringConcatenationIndentation</code></li>
<li>Add support for disabling linters on specific lines</li>
</ul>
<h2>0.21.1</h2>
<ul>
<li>Fix handling of Slim documents with parse error</li>
</ul>
<h2>0.21.0</h2>
<ul>
<li>Add <code>linter</code> field to <code>JSONReporter</code></li>
<li>Add <code>EmbeddedEngines</code> linter</li>
</ul>
<!-- raw HTML omitted -->
</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/sds/slim-lint/commit/c2b8e7af34075a1befaf2a22e0655340ba1a4707"><code>c2b8e7a</code></a>
Cut version 0.28.0 (<a
href="https://redirect.github.com/sds/slim-lint/issues/180">#180</a>)</li>
<li><a
href="https://github.com/sds/slim-lint/commit/2c8b59bdd3a679360ad5ed009ccad847cd30a763"><code>2c8b59b</code></a>
Add Tag linter (<a
href="https://redirect.github.com/sds/slim-lint/issues/179">#179</a>)</li>
<li>See full diff in <a
href="https://github.com/sds/slim-lint/compare/v0.27.0...v0.28.0">compare
view</a></li>
</ul>
</details>
<br />

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</details>

govuk-components.gemspec

@@ -44,6 +44,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("sass")
   spec.add_development_dependency("sassc", "~> 2.4.0")
   spec.add_development_dependency("slim", "~> 5.2.0")
-  spec.add_development_dependency("slim_lint", "~> 0.27.0")
+  spec.add_development_dependency("slim_lint", "~> 0.28.0")
   spec.add_development_dependency("webrick", "~> 1.8.1")
 end