pin karma-webpack to 4.0.0-beta.0 to fix perf regression

so karma-webpack 3.0.5 is 4x slower than 4.0.0-beta.0 for webpacking for unknown reasons.
However, 4.0.0-rc.1 and 4.0.0-rc.2 fail with a different error. So: pinned for now

package.json

@@ -122,7 +122,7 @@
     "karma-sourcemap-loader": "^0.3.7",
     "karma-spec-reporter": "0.0.31",
     "karma-summary-reporter": "^1.5.1",
-    "karma-webpack": "^3.0.5",
+    "karma-webpack": "4.0.0-beta.0",
     "matrix-mock-request": "^1.2.0",
     "matrix-react-test-utils": "^0.2.0",
     "minimist": "^1.2.0",