Update Circle CI Node Version to 7.9.0

The purpose of this is to be able to land facebookarchive#397 with some test coverage.
In its current form it is too specific in its implementation and only
works on a specific version of Node. In the future we'll want to make that
more abstract and general so that it can work on ranges and other major
versions.

circle.yml

@@ -1,6 +1,6 @@
 machine:
   node:
-    version: 6.1.0
+    version: 7.9.0
   environment:
     PATH: "${PATH}:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"