ci: temporarily switch to Node v11.10 to fix tests

There's an upcoming fix in Node, so we're doing this only temporarily to prevent CI failure.

jestjs/jest#8069

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: node_js
 node_js:
-  - stable
+  - "11.10"
 cache: yarn
 script:
   - yarn test