polish osx platform for travis

.travis.yml

-os:
-  - linux
-  - osx
+notifications:
+  email:
+    - dev@rocketmq.incubator.apache.org
+    on_success: change
+    on_failure: always
 language: java
-jdk:
-  - oraclejdk8
-  - oraclejdk7
-  - openjdk7
 matrix:
   include:
-    - os: osx
-      osx_image: xcode8
+  # On OSX, run with default JDK only.
+  - os: osx
+  # On Linux, run with specific JDKs only.
+  - os: linux
+    env: CUSTOM_JDK="oraclejdk8"
+  - os: linux
+    env: CUSTOM_JDK="oraclejdk7"
+  - os: linux
+    env: CUSTOM_JDK="openjdk7"
+
+before_install:
+  - echo 'MAVEN_OPTS="$MAVEN_OPTS -Xmx1024m -XX:MaxPermSize=512m -XX:+BytecodeVerificationLocal"' >> ~/.mavenrc
+  - cat ~/.mavenrc
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then export JAVA_HOME=$(/usr/libexec/java_home); fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then jdk_switcher use "$CUSTOM_JDK"; fi
+#
+#os:
+#  - linux
+#  - osx
+#jdk:
+#  - oraclejdk8
+#  - oraclejdk7
+#  - openjdk7
+
 
 #script:
 #  - travis_retry mvn --batch-mode clean apache-rat:check