Don't download from (unreliable) maven.org (#6348)

Don't download from (unreliable) maven.org (#6348)

Summary:
I set up a mirror of our Java deps on github so we can download
them through github URLs rather than maven.org, which is proving
terribly unreliable from Travis builds.

Also sanitized calls to curl, so they are easier to read and
appropriately fail on download failure.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6348

Test Plan: CI

Differential Revision: D19633621

Pulled By: pdillinger

fbshipit-source-id: 7eb3f730953db2ead758dc94039c040f406790f3