.travis.yml: Try to install zstd on Travis-CI linux, osx environments
.travis.yml: Try to install zstd on Travis-CI linux, osx environments

Not sure if the zstd brew formula is new enough or not.

The Travis-CI ubuntu environment is not new enough to have an up-to-date
libzstd-dev package.

farsightsec/mtbl,farsightsec/mtbl