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

diff --git a/.travis.yml b/.travis.yml
index a6c3dcc..e3b327b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,7 +12,7 @@
 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)
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