[ci] take #2, improve build times by saving TerserPlugin cache (#6882)

The problem with the previous solution was that the `npm ci` command
nuke the `node_modules` folder, including the `.cache` that was used by
default. By moving the cache out of `node_modules`  we get to both run
`npm ci` and accelerate `TerserPlugin`

.gitignore

@@ -77,3 +77,4 @@ celerybeat.pid
 geckodriver.log
 ghostdriver.log
 testCSV.csv
+.terser-plugin-cache/

.travis.yml

@@ -95,4 +95,4 @@ cache:
     - ~/.npm
     - ~/.cache
     - ~/.travis_cache/
-    - superset/assets/node_modules/.cache/terser-webpack-plugin/
+    - superset/assets/.terser-plugin-cache/

superset/assets/webpack.config.js

@@ -270,7 +270,7 @@ const config = {
 if (!isDevMode) {
   config.optimization.minimizer = [
     new TerserPlugin({
-      cache: true,
+      cache: '.terser-plugin-cache/',
       parallel: true,
       extractComments: true,
     }),