fix travis failure for ccache command not found
fix travis failure for ccache command not found

Summary:
Travis don't have ccache installed on Mac. Only run the ccache command when it exists.
https://docs.travis-ci.com/user/caching/#ccache-cache
Closes https://github.com/facebook/rocksdb/pull/2990

Differential Revision: D6028837

Pulled By: yiwu-arbug

fbshipit-source-id: 1e2d1c7f37be2c73773258c1fd5f24eebe7a06c6

Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb