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