(maint) Remove dead dependency

The sys-admin gem is no longer used.
(FACT-567) Support bundler workflow on x64

This commit moves windows gem dependencies back into the Gemfile
because of problems trying to express dependencies for x86 and x64
simultaneously. For example, in ruby 1.9, rubygems does not know about
the x64_mingw platform. Also win32console is only a dependency in ruby
1.9 x86, but not in ruby 2 x86 or x64.
Merge pull request #692 from joshcooper/ticket/facter-2/FACT-567-support-bundler-x64

(FACT-567) support bundler x64