chore(deps): Bump middleman from 339477e to 18e328a (#1445)

Bumps [middleman](https://github.com/middleman/middleman) from `339477e` to `18e328a`.
- [Commits](middleman/middleman@339477e...18e328a)

---
updated-dependencies:
- dependency-name: middleman
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -9,7 +9,7 @@ GIT
 
 GIT
   remote: https://github.com/middleman/middleman.git
-  revision: 339477ee96d788d3d9cf6d518a3395279f72a0af
+  revision: 18e328a14e7b816fb39304bde707b6f1b7e799cd
   specs:
     middleman (4.5.1)
       middleman-cli (= 4.5.1)