Downgrade node temporarily (#7333)

To fix jest related build failure as recommended at https://github.com/facebook/jest/issues/8069#issuecomment-470307590

.travis.yml

@@ -44,7 +44,7 @@ before_script:
   - if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6'; fi
 
 before_install:
- - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then nvm install stable ; fi
+ - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then nvm install 11.10.1 ; fi
 
 after_success:
 - bash <(curl -s https://codecov.io/bash)