(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.
I think we want '~> 0.8.4' (for both x86-mingw32 and x64-mingw32) so that when djberg releases a new gem, we don't have to update this file again.