ghc-prim: Bump version

Authored by bgamari on May 20 2018, 6:47 PM.

Description

ghc-prim: Bump version

unpackClosure#'s behavior and type has changed. This caused a CPP guard
in the new ghc-heap package to fail when bootstrapping with GHC 8.4.

Test Plan: Validate bootstrapping with GHC 8.4

Reviewers: RyanGlScott

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4716