Bump mdl from 0.8.0 to 0.9.0 (#653)

Bumps [mdl](https://github.com/mivok/markdownlint) from 0.8.0 to 0.9.0.
- [Release notes](https://github.com/mivok/markdownlint/releases)
- [Changelog](https://github.com/markdownlint/markdownlint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mivok/markdownlint/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile

@@ -5,7 +5,7 @@ source "https://rubygems.org"
 gemspec
 
 gem "chandler", "0.9.0"
-gem "mdl", "0.8.0"
+gem "mdl", "0.9.0"
 gem "minitest", "~> 5.11"
 gem "pry", "0.12.2"
 gem "rake", "~> 13.0"

Gemfile.lock

@@ -21,15 +21,15 @@ GEM
     kramdown (2.1.0)
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
-    mdl (0.8.0)
+    mdl (0.9.0)
       kramdown (~> 2.0)
       kramdown-parser-gfm (~> 1.0)
       mixlib-cli (~> 2.1, >= 2.1.1)
       mixlib-config (>= 2.2.1, < 4)
     method_source (0.9.2)
     minitest (5.14.0)
-    mixlib-cli (2.1.1)
-    mixlib-config (3.0.1)
+    mixlib-cli (2.1.5)
+    mixlib-config (3.0.6)
       tomlrb
     multipart-post (2.1.1)
     netrc (0.11.0)
@@ -67,7 +67,7 @@ GEM
       json (>= 1.8, < 3)
       simplecov-html (~> 0.10.0)
     simplecov-html (0.10.2)
-    tomlrb (1.2.8)
+    tomlrb (1.2.9)
     unicode-display_width (1.6.1)
     yard (0.9.24)
 
@@ -78,7 +78,7 @@ DEPENDENCIES
   bundler (~> 2.0)
   byebug!
   chandler (= 0.9.0)
-  mdl (= 0.8.0)
+  mdl (= 0.9.0)
   minitest (~> 5.11)
   pry (= 0.12.2)
   rake (~> 13.0)