Bump recommended hackney to ~> 1.15.2

Due to unannounced breaking change in OTP version 22.1 hackney below the version 1.15.2 stopped working.

Older versions are throwing {:option, :server_only, :honor_cipher_order} on OTP >= 22.1 and 1.15.2 fixes this issue.

Reference:
https://github.com/benoitc/hackney/issues/591
https://github.com/edgurgel/httpoison/issues/393