update shelljs to get rid of warnings on build

package.json

@@ -103,7 +103,7 @@
     "selenium-server": "2.53.1",
     "semver": "^5.3.0",
     "serviceworker-webpack-plugin": "^1.0.0",
-    "shelljs": "^0.7.4",
+    "shelljs": "^0.8.4",
     "sinon": "^2.1.0",
     "sinon-chai": "^2.8.0",
     "stylelint": "^13.6.1",

yarn.lock

@@ -7842,9 +7842,10 @@ shebang-regex@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/shebang-regex/-/shebang-regex-1.0.0.tgz#da42f49740c0b42db2ca9728571cb190c98efea3"
 
-shelljs@^0.7.4:
-  version "0.7.8"
-  resolved "https://registry.yarnpkg.com/shelljs/-/shelljs-0.7.8.tgz#decbcf874b0d1e5fb72e14b164a9683048e9acb3"
+shelljs@^0.8.4:
+  version "0.8.4"
+  resolved "https://registry.yarnpkg.com/shelljs/-/shelljs-0.8.4.tgz#de7684feeb767f8716b326078a8a00875890e3c2"
+  integrity sha512-7gk3UZ9kOfPLIAbslLzyWeGiEqx9e3rxwZM0KE6EL8GlGwjym9Mrlx5/p33bWTu9YG6vcS4MBxYZDHYr5lr8BQ==
   dependencies:
     glob "^7.0.0"
     interpret "^1.0.0"