use xcode9.3 to force travis ci to java 8 (#64)

Author: andxu

.travis.yml

@@ -7,6 +7,8 @@ os:
 - linux
 - osx
 
+osx_image: xcode9.3
+
 before_install:
 - if [ $TRAVIS_OS_NAME == "linux" ]; then
       export CXX="g++-4.9" CC="gcc-4.9" DISPLAY=:99.0;