Bump middleman from 4.2.1 to 4.3.1

Bumps [middleman](https://github.com/middleman/middleman) from 4.2.1 to 4.3.1.
- [Release notes](https://github.com/middleman/middleman/releases)
- [Changelog](https://github.com/middleman/middleman/blob/v4.3.1/CHANGELOG.md)
- [Commits](middleman/middleman@v4.2.1...v4.3.1)

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -19,8 +19,6 @@ GEM
       coffee-script-source
       execjs
     coffee-script-source (1.12.2)
-    compass-import-once (1.0.5)
-      sass (>= 3.2, < 3.5)
     concurrent-ruby (1.1.4)
     contracts (0.13.0)
     dotenv (2.6.0)
@@ -33,39 +31,39 @@ GEM
     fast_blank (1.0.0)
     fastimage (2.1.5)
     ffi (1.9.25)
-    haml (4.0.7)
+    haml (5.0.4)
+      temple (>= 0.8.0)
       tilt
     hamster (3.0.0)
       concurrent-ruby (~> 1.0)
     hashie (3.6.0)
     htmlcompressor (0.2.0)
     http_parser.rb (0.6.0)
-    i18n (0.7.0)
-    kramdown (1.14.0)
+    i18n (0.9.5)
+      concurrent-ruby (~> 1.0)
+    kramdown (1.17.0)
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
     memoist (0.16.0)
-    middleman (4.2.1)
+    middleman (4.3.1)
       coffee-script (~> 2.2)
-      compass-import-once (= 1.0.5)
       haml (>= 4.0.5)
       kramdown (~> 1.2)
-      middleman-cli (= 4.2.1)
-      middleman-core (= 4.2.1)
-      sass (>= 3.4.0, < 4.0)
+      middleman-cli (= 4.3.1)
+      middleman-core (= 4.3.1)
     middleman-aria_current (0.1.1)
       middleman-core (~> 4.0)
     middleman-autoprefixer (2.10.0)
       autoprefixer-rails (>= 9.1.4)
       middleman-core (>= 3.3.3)
-    middleman-cli (4.2.1)
+    middleman-cli (4.3.1)
       thor (>= 0.17.0, < 2.0)
-    middleman-core (4.2.1)
+    middleman-core (4.3.1)
       activesupport (>= 4.2, < 5.1)
       addressable (~> 2.3)
       backports (~> 3.6)
-      bundler (~> 1.1)
+      bundler
       contracts (~> 0.13.0)
       dotenv
       erubis
@@ -74,15 +72,15 @@ GEM
       fastimage (~> 2.0)
       hamster (~> 3.0)
       hashie (~> 3.4)
-      i18n (~> 0.7.0)
+      i18n (~> 0.9.0)
       listen (~> 3.0.0)
       memoist (~> 0.14)
       padrino-helpers (~> 0.13.0)
       parallel
       rack (>= 1.4.5, < 3)
-      sass (>= 3.4)
+      sassc (~> 2.0)
       servolux
-      tilt (~> 2.0)
+      tilt (~> 2.0.9)
       uglifier (~> 3.0)
     middleman-livereload (3.4.6)
       em-websocket (~> 0.5.1)
@@ -134,7 +132,7 @@ GEM
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     temple (0.8.0)
-    thor (0.20.0)
+    thor (0.20.3)
     thread_safe (0.3.6)
     tilt (2.0.9)
     tzinfo (1.2.5)