Use acorn@7.1.1 (#1435)

This PR resolves security advisories of the form:

```
┌───────────────┬──────────────────────────────────────────────────────────────┐
│ moderate      │ Regular Expression Denial of Service                         │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Package       │ acorn                                                        │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Patched in    │ >=5.7.4 <6.0.0 || >=6.4.1 <7.0.0 || >=7.1.1                  │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Dependency of │ eslint                                                       │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Path          │ eslint > espree > acorn                                      │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ More info     │ https://www.npmjs.com/advisories/1488                        │
└───────────────┴──────────────────────────────────────────────────────────────┘
```

yarn.lock

@@ -1196,9 +1196,9 @@ acorn@^6.0.1, acorn@^6.0.7:
   integrity sha512-ZVA9k326Nwrj3Cj9jlh3wGFutC2ZornPNARZwsNYqQYgN0EsV2d53w5RN/co65Ohn4sUAUtb1rSUAOD6XN9idA==
 
 acorn@^7.1.0:
-  version "7.1.0"
-  resolved "https://registry.yarnpkg.com/acorn/-/acorn-7.1.0.tgz#949d36f2c292535da602283586c2477c57eb2d6c"
-  integrity sha512-kL5CuoXA/dgxlBbVrflsflzQ3PAas7RYZB52NOm/6839iVYJgKMJ3cQJD+t2i5+qFa8h3MDpEOJiS64E8JLnSQ==
+  version "7.1.1"
+  resolved "https://registry.yarnpkg.com/acorn/-/acorn-7.1.1.tgz#e35668de0b402f359de515c5482a1ab9f89a69bf"
+  integrity sha512-add7dgA5ppRPxCFJoAGfMDi7PIBXq1RtGo7BhbLaxwrXPOmw8gq48Y9ozT01hUKy9byMjlR20EJhu5zlkErEkg==
 
 aes-js@3.0.0:
   version "3.0.0"