add travis-ci script for java wrapper

scripts/travis_java_script.sh

@@ -0,0 +1,6 @@
+# Test java package of xgboost
+cd java
+./create_wrap.sh
+cd xgboost4j
+mvn clean install -DskipTests=true
+mvn test

scripts/travis_script.sh

@@ -26,3 +26,8 @@ if [ ${TASK} == "python-package" ]; then
     make all CXX=${CXX} || exit -1
     nosetests tests/python || exit -1
 fi
+
+if [ ${TASK} == "java-package" ]; then
+    make java CXX=${CXX} || exit -1
+    scripts/travis_java_script.sh || exit -1
+fi