Use GCC 9 on Travis CI
Use GCC 9 on Travis CI

PiperOrigin-RevId: 249899128

cmumford/leveldb,dwing4g/leveldb,cmumford/leveldb,dwing4g/leveldb,google/leveldb,google/leveldb,dwing4g/leveldb,bitcoin-core/leveldb,bitcoin-core/leveldb,matehat/leveldb,matehat/leveldb,matehat/leveldb,bitcoin-core/leveldb