remove -Xms500m -Xmx1g -XX:MaxPermSize=256m temporarily

Author: William Guo <guoyp@apache.org>

Closes #544 from guoyuepeng/fix_maven_memory_opt.
diff --git a/.travis.yml b/.travis.yml
index 5f21f27..57405c6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -43,6 +43,6 @@
 # keep 30, need change according to ci logs.
 script:
   - export MAVEN_SKIP_RC=true
-  - export MAVEN_OPTS="-Xms500m -Xmx1g -XX:MaxPermSize=256m -Dorg.slf4j.simpleLogger.defaultLogLevel=warn -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn"
+  - export MAVEN_OPTS=" -Dorg.slf4j.simpleLogger.defaultLogLevel=warn -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn"
   - mvn -B clean test -Dlogging.level.org.springframework=WARN