[chore] Update ci to use an older node 11 version

See jestjs/jest#8069
Also use pure-lockfile to be sure to always keep the good deps

Author: seblucas

.travis.yml

@@ -2,11 +2,14 @@ language: node_js
 
 node_js:
   - "10"
-  - "11"
+  - "11.10.1"
 
 before_install:
   - npm install -g yarn@1.12.3
 
+install:
+  - yarn install --pure-lockfile
+
 script:
   - yarn lint
   - yarn test:unit --coverage --ci