The Travis CI configuration updates reflect the following changes:
* Container-based builds (sudo: false) have been removed.
  https://changelog.travis-ci.com/the-container-based-build-environment-is-fully-deprecated-84517
* Ubuntu Xenial (16.04) is available as a base image.
  https://blog.travis-ci.com/2018-11-08-xenial-release
* Homebrew now has a dedicated DSL.
  https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-on-os-x

To take full advantage of VM resources, CI builds now use Ninja
https://ninja-build.org/ instead of Make.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=227611641

Equivalent of
db082d2cd6

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211467181

Fixes https://github.com/google/leveldb/issues/466

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189425354

This aligns the Travis CI configuration with google/crc32c and
google/snappy, to simplify maintenance.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188547648

This allows for continuous integration builds by travis-ci.org.
More information at https://docs.travis-ci.com/user/languages/cpp