fix: Run xvf ni foreground

Author: 1ambda

.travis.yml

@@ -129,7 +129,7 @@ before_script:
   - echo "export SPARK_HOME=`pwd`/spark-$SPARK_VER-bin-hadoop$HADOOP_VER" > conf/zeppelin-env.sh
   - echo "export ZEPPELIN_HELIUM_REGISTRY=helium" >> conf/zeppelin-env.sh
   - tail conf/zeppelin-env.sh
-  - if [[ -n $TEST_MODULES ]]; then export DISPLAY=:99.0; sh -e /etc/init.d/xvfb start &; fi
+  - if [[ -n $TEST_MODULES ]]; then export DISPLAY=:99.0; sh -e /etc/init.d/xvfb start; sleep 3; fi
   - if [[ -n $TEST_MODULES ]]; then echo "MAVEN_OPTS='-Xms1024M -Xmx2048M -XX:MaxPermSize=1024m -XX:-UseGCOverheadLimit -Dorg.slf4j.simpleLogger.defaultLogLevel=info'" > ~/.mavenrc; fi
 
 script: