This is a RubyGems bug on Windows. It can be fixed by
updating RubyGems or updating to
Ruby 2.3.2. I'm waiting for the latter to be available
for Windows to update reapack-index's installer.
In the meantime the simplest workaround is to download this file manually:
https://rubygems.org/downloads/reapack-index-1.1rc5.gem and run "gem install Path\To\reapack-index-1.1rc5.gem" from a command prompt.