Bumps terser-webpack-plugin to 2.3.4

We're seeing occasional "Error: Call retries were exceeded" exception
with Terser, which should be resolved by
webpack/terser-webpack-plugin@abfd950,
first included in 2.3.4

Signed-off-by: Tyler Smalley <[email protected]>

Author: tylersmalley

package.json

@@ -262,7 +262,7 @@
     "style-loader": "0.23.1",
     "symbol-observable": "^1.2.0",
     "tar": "4.4.13",
-    "terser-webpack-plugin": "^2.1.2",
+    "terser-webpack-plugin": "^2.3.4",
     "thread-loader": "^2.1.3",
     "tinygradient": "0.4.3",
     "tinymath": "1.2.1",