fix(deps): update dependency acorn to v7 (#748)

Author: DominicKramer

package.json

@@ -50,7 +50,7 @@
   "dependencies": {
     "@google-cloud/common": "^2.0.0",
     "@sindresorhus/is": "^1.0.0",
-    "acorn": "^6.0.0",
+    "acorn": "^7.0.0",
     "coffeescript": "^2.0.0",
     "console-log-level": "^1.4.0",
     "extend": "^3.0.2",