Bump loofah from 2.2.3 to 2.4.0 in /week4/sample_app

Bumps [loofah](https://github.com/flavorjones/loofah) from 2.2.3 to 2.4.0.
- [Release notes](https://github.com/flavorjones/loofah/releases)
- [Changelog](https://github.com/flavorjones/loofah/blob/master/CHANGELOG.md)
- [Commits](flavorjones/loofah@v2.2.3...v2.4.0)

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

Author: dependabot[bot]

week4/sample_app/Gemfile.lock

@@ -57,7 +57,7 @@ GEM
       execjs
     coffee-script-source (1.12.2)
     concurrent-ruby (1.1.5)
-    crass (1.0.4)
+    crass (1.0.6)
     erubi (1.8.0)
     execjs (2.7.0)
     ffi (1.11.1)
@@ -90,7 +90,7 @@ GEM
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
       ruby_dep (~> 1.2)
-    loofah (2.2.3)
+    loofah (2.4.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     lumberjack (1.0.13)