Switch back to the official xgboost-predictor release after it supports xgboost 1.0.0

[mengxr]

See https://github.com/combust/mleap/pull/686#discussion_r434005552

[ancasarb]

@voganrc @talalryz if i'm not mistaken, @lucagiovagnoli already made this switch in master, right? could i please ask you to double check if we can close this issue? thank you!