Fixed Node v12 failing build on Travis

no issue

- Node v12 goes into LTS on 2019-10-22 (https://github.com/nodejs/Release#release-schedule)
- It is not a recommended version but we want to start testing it early
- Added '--ignore-engines' option to `install` step to make it possible
to run v12 build without adding new entry in Ghost's package.json

.travis.yml

@@ -21,10 +21,10 @@ matrix:
   - node_js: '12'
   fast_finish: true
 install:
-- yarn
+- if [ "$TRAVIS_NODE_VERSION" == "12" ]; then yarn --ignore-engines; else yarn; fi
 before_script:
 - if [ $DB == "mysql" ]; then mysql -e 'create database ghost_testing'; fi
-- if [ "$DB" == "sqlite3" ]; then yarn add --force sqlite3; fi
+- if [ "$DB" == "sqlite3" ]; then yarn add --ignore-engines --force sqlite3; fi
 script: |
   if [ "$TEST_SUITE" == "lint" ]; then
     yarn lint