chore(ci): run jest with 2 workers on CI

Author: SimenB

.travis.yml

@@ -14,7 +14,7 @@ script:
 - yarn commitlint --from="$TRAVIS_BRANCH" --to="$TRAVIS_COMMIT"
 - yarn commitlint --from=$TRAVIS_COMMIT
 - yarn prettylint
-- yarn test --coverage
+- yarn test --coverage --maxWorkers 2
 after_script: greenkeeper-lockfile-upload
 jobs:
   include: