Bump acorn from 6.1.1 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 6.1.1 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.1.1...6.4.2)

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

Author: dependabot[bot]

yarn.lock

@@ -671,8 +671,8 @@ acorn-dynamic-import@^4.0.0:
   resolved "https://registry.yarnpkg.com/acorn-dynamic-import/-/acorn-dynamic-import-4.0.0.tgz#482210140582a36b83c3e342e1cfebcaa9240948"
 
 acorn@^6.0.5:
-  version "6.1.1"
-  resolved "https://registry.yarnpkg.com/acorn/-/acorn-6.1.1.tgz#7d25ae05bb8ad1f9b699108e1094ecd7884adc1f"
+  version "6.4.2"
+  resolved "https://registry.yarnpkg.com/acorn/-/acorn-6.4.2.tgz#35866fd710528e92de10cf06016498e47e39e1e6"
 
 ajv-errors@^1.0.0:
   version "1.0.1"