Revert "do not hardcode protobuf compiler path. use java 7"

This reverts commit 044bc3d7d1214e8ccb1433457a63c4f9ee727760.
Revert "Revert "do not hardcode protobuf compiler path. use java 7""

This reverts commit 5edc6af11fa8476b6fe532e9877e1918001745e8.