Ask travis to use JDK 7
Ask travis to use JDK 7

Summary:
yhchiang This may or may not help
Closes https://github.com/facebook/rocksdb/pull/1385

Differential Revision: D4098424

Pulled By: yhchiang

fbshipit-source-id: 9f9782e

vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb