fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-LODASH-590103

Author: snyk-bot

package.json

@@ -37,7 +37,7 @@
     "highlight.js": "^9.12.0",
     "jquery": "^3.0.0",
     "lcov-parse": "^1.0.0",
-    "lodash": "^4.17.4",
+    "lodash": "^4.17.20",
     "nib": "^1.1.0",
     "riot": "^2.2.4"
   }

yarn.lock

@@ -2094,6 +2094,11 @@ lodash@^4.14.0:
   version "4.17.4"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.4.tgz#78203a4d1c328ae1d86dca6460e369b57f4055ae"
 
+lodash@^4.17.20:
+  version "4.17.21"
+  resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.21.tgz#679591c564c3bffaae8454cf0b3df370c3d6911c"
+  integrity sha512-v2kDEe57lecTulaDIuNTPy3Ry4gLGJ6Z1O3vE1krgXZNrsQ+LFTGHVxVjcXPs17LhbZVGedAJv8XZ1tvj5FvSg==
+
 lodash@^4.17.4:
   version "4.17.15"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.15.tgz#b447f6670a0455bbfeedd11392eff330ea097548"