(PUP-1283) Upgrade win32-service gem

This upgrades win32-service from 0.7.2 (native compiled gem) to
0.8.4 (FFI-ed gem), which is a step in getting puppet to run on x64
ruby.
(PUP-1283) Relax win32-service constraint

Previously we were requiring an exact version match for the win32-service
gem. However, the ffi version of the gem has a regression and we expect a
new version of the gem to be released soon.

This commit relaxes the dependency so that we don't have to revisit this
issue in the near future.
Merge branch 'pr/2828'

* pr/2828:
  (PUP-1283) Relax win32-service constraint
  (PUP-1283) Upgrade win32-service gem