Use slightly older Node.js versions

Due to a limit to header sizes in the latest security releases of
Node.js, combined with Elasticsearch's default of sending lots of
deprecation warning errors as headers, we need to use slightly older
versions of Node.js until either Elasticsearch offers more configuration
options, or Node.js releases a CLI option for the header limit.

See #337 for details

.travis.yml

@@ -4,8 +4,8 @@ dist: trusty
 notifications:
   email: false
 node_js:
-  - 8
-  - 10
+  - 8.13.0
+  - 10.13.0
 matrix:
   fast_finish: true
 env: