travis: clang-3.6 -> clang-4.0
travis: clang-3.6 -> clang-4.0

Summary:
just realised when I updated the .travis.yml to trusty the llvm repo was still precise. Update this and clang-4.0.
Closes https://github.com/facebook/rocksdb/pull/2127

Differential Revision: D4869427

Pulled By: sagar0

fbshipit-source-id: b7f906b6fac28e60cacc6a1f1959d6acf8269906

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