Update dependency clean-css to v5

package.json

@@ -10,7 +10,7 @@
     "node": ">= 8.6.0"
   },
   "dependencies": {
-    "clean-css": "4.2.1",
+    "clean-css": "5.1.2",
     "express": "4.17.1",
     "express-history-api-fallback": "2.2.1",
     "highland": "2.13.4",

yarn.lock

@@ -736,7 +736,14 @@ class-utils@^0.3.5:
     isobject "^3.0.0"
     static-extend "^0.1.1"
 
-[email protected], clean-css@^4.2.1:
+[email protected]:
+  version "5.1.2"
+  resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-5.1.2.tgz#6ea0da7286b4ddc2469a1b776e2461a5007eed54"
+  integrity sha512-QcaGg9OuMo+0Ds933yLOY+gHPWbxhxqF0HDexmToPf8pczvmvZGYzd+QqWp9/mkucAOKViI+dSFOqoZIvXbeBw==
+  dependencies:
+    source-map "~0.6.0"
+
+clean-css@^4.2.1:
   version "4.2.1"
   resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.2.1.tgz#2d411ef76b8569b6d0c84068dabe85b0aa5e5c17"
   dependencies: