add gradle wrapper jar file
Merge commit 'c239e06a75f50a5b1bef953fbc157b0b81c35b5c'
Revert "try to remove gradlew and let Travis use local Gradle."

This reverts commit 14bfc381ccf9ba55a19ebb1db987fab43e370a3d.
Revert "change gradle to 2.0"

This reverts commit bed8c4e7424599e938019937f1fd322bf08520cf.