chore(server): Set node version to 8 for Travis CI

Yarn is failing with node 9 because graphql-js v14.0.0 removed support for node 9 in the "engines" field of package.json.

Looks like this will be sorted out in the next release (graphql/graphql-js#1508) but for now it is easiest to use node v8 to make the build pass.

.travis.yml

@@ -6,7 +6,7 @@ git:
 
 language: node_js
 node_js:
-  - "9"
+  - "8"
 
 cache: yarn