test: update karma npm from 3.1.4 to 6.4.2

karma 3.x was stable in 2018, but 6.x is now the latest.

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

actioncable/package.json

@@ -24,11 +24,11 @@
   },
   "homepage": "https://rubyonrails.org/",
   "devDependencies": {
-    "@rollup/plugin-node-resolve": "^11.0.1",
     "@rollup/plugin-commonjs": "^19.0.1",
+    "@rollup/plugin-node-resolve": "^11.0.1",
     "eslint": "^4.3.0",
     "eslint-plugin-import": "^2.7.0",
-    "karma": "^3.1.1",
+    "karma": "^6.4.2",
     "karma-chrome-launcher": "^2.2.0",
     "karma-qunit": "^2.1.0",
     "karma-sauce-launcher": "^1.2.0",