Use :cmd backend for Windows platform (#4)

Seems `:exec` assumes Linux environment.

So, this patch uses `:cmd` as backend on Windows.

Author: Watson1978

lib/fluent/plugin/fluent_package_update_checker.rb

@@ -24,7 +24,7 @@ def initialize(options={})
             @logger.error "Failed to load #{ENV["FLUENT_PACKAGE_CONFIG"] || DEFAULT_PACKAGE_CONFIG_PATH}"
           end
           @host_os = RbConfig::CONFIG['host_os']
-          Specinfra.configuration.backend = :exec
+          Specinfra.configuration.backend = windows? ? :cmd : :exec
           @os_info = Specinfra.backend.os_info
         end