zstd 1.4.4

This release updates the upstream zstd version to 1.4.4 (#73 thanks to @dbenamydd)

It includes improvement for this library (Go zstd library):

• Adding buffer pools for stream decompression @dangermike (#69)

CI / Documentation related PRs:

• [doc] Update upstream zstd link @ofek (#71)
• [ci] Add CircleCI as CI and remove Travis @Viq111 (#68)