Update acorn dependency for security updates. (dollarshaveclub#114)

es-check pins the acorn dependency to 6.1.1, which has a security issue:
https://www.npmjs.com/advisories/1488

This updates and loosens the version constraint to a patched version.

package.json

@@ -43,7 +43,7 @@
     "nyc": "^14.1.1"
   },
   "dependencies": {
-    "acorn": "6.1.1",
+    "acorn": "^6.4.1",
     "caporal": "1.3.0",
     "glob": "^7.1.2"
   },

yarn.lock

@@ -102,14 +102,15 @@ acorn-jsx@^4.1.1:
   dependencies:
     acorn "^5.0.3"
 
-[email protected]:
-  version "6.1.1"
-  resolved "https://registry.yarnpkg.com/acorn/-/acorn-6.1.1.tgz#7d25ae05bb8ad1f9b699108e1094ecd7884adc1f"
-
 acorn@^5.0.3, acorn@^5.6.0:
   version "5.7.3"
   resolved "https://registry.yarnpkg.com/acorn/-/acorn-5.7.3.tgz#67aa231bf8812974b85235a96771eb6bd07ea279"
 
+acorn@^6.4.1:
+  version "6.4.1"
+  resolved "https://registry.yarnpkg.com/acorn/-/acorn-6.4.1.tgz#531e58ba3f51b9dacb9a6646ca4debf5b14ca474"
+  integrity sha512-ZVA9k326Nwrj3Cj9jlh3wGFutC2ZornPNARZwsNYqQYgN0EsV2d53w5RN/co65Ohn4sUAUtb1rSUAOD6XN9idA==
+
 add-stream@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/add-stream/-/add-stream-1.0.0.tgz#6a7990437ca736d5e1288db92bd3266d5f5cb2aa"