(PUP-2828) Select the branch of puppet-win32-ruby to install

Previously, we were always installing ruby 1.9.3 on Windows, which
contains the gem sets for 3.2.x to 3.6.x.

This commit updates the setup step to install ruby from the 1.9.3-x86
branch (by default). If the WIN32_RUBY_ARCH environment variable is
specified, then it will install ruby from the 2.0.0-x64 branch instead.