Bump acorn from 4.0.13 to 7.1.1 in /materialjs

Bumps [acorn](https://github.com/acornjs/acorn) from 4.0.13 to 7.1.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/commits/7.1.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

materialjs/package.json

@@ -8,7 +8,7 @@
   "repository": "google/material-design-lite",
   "main": "dist/material.min.js",
   "devDependencies": {
-    "acorn": "^4.0.3",
+    "acorn": "^7.1.1",
     "babel-core": "^6.20.0",
     "babel-preset-es2015": "^6.18.0",
     "browser-sync": "^2.2.3",