Add quiet mode for Maven build since Dubbo 2.6.7 (apache#4559)

Author: mercyblitz

.travis.yml

@@ -12,7 +12,7 @@ cache:
 install: true
 
 script:
-  - travis_wait 30 ./mvnw clean install -DskipTests=false -Dcheckstyle.skip=false -Drat.skip=false -Dmaven.javadoc.skip=true
+  - travis_wait 30 ./mvnw clean install -q -DskipTests=false -Dcheckstyle.skip=false -Drat.skip=false -Dmaven.javadoc.skip=true
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)