fix: add chownr to snyk ignore file

Chownr has a recently reported issue to snyk, though the issue itself
has been known for over a year. isaacs/chownr#14
This issue has been introduced via sharp via tar and tar-fs npm packages

We'll continue to follow this issue on the chownr repo. And I've added
a comment to the issue thread.
isaacs/chownr#14 (comment)

.snyk

@@ -22,3 +22,7 @@ ignore:
     - '*':
       reason: "extend and request have updated, waiting on twillio and a few other packages to update to the latest requestpackage"
       expires: '2018-08-26T20:23:03.274Z'
+  'npm:chownr:20180731':
+    - '*':
+      reason: "Known bug https://github.com/isaacs/chownr/issues/14, unknown time to patch"
+      expires: '2018-08-30T20:23:03.274Z'