travis: reduce the number of travis builders
travis: reduce the number of travis builders

Summary:
This collapses all the "platform dependent" tests into a single travis
builder in an effort to reduce overall CI times. These builds currently
take a combined 21-23 minutes, but each one has to compile the library,
so combining them should yield some time savings (5-10 minutes).

Unfortunately the other builders don't duplicate work, so combining
them is unlikely to provide benefit.
Closes https://github.com/facebook/rocksdb/pull/2306

Differential Revision: D5147850

Pulled By: yiwu-arbug

fbshipit-source-id: d947dc8b9f49639fe22f3c8ab9a82a8d730ddddf

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