travis: add osx
travis: add osx

Level/rocksdb,Level/leveldown,Level/leveldown,MathieuCassagne/leveldown,ralphtheninja/leveldown,Level/rocksdb,ralphtheninja/leveldown,MathieuCassagne/leveldown,Level/rocksdb,MathieuCassagne/leveldown,MathieuCassagne/leveldown,ralphtheninja/leveldown,MathieuCassagne/leveldown,ralphtheninja/leveldown,ralphtheninja/leveldown,Level/leveldown