Use where on Windows instead of which (#299) (#300)

If the call is on Windows, it uses `where` instead of `which` and correctly sends the output to NUL so that it does not show unnecessary message on the installer's output. This fixes the incorrect/ misleading message shown on Windows currently.

Author: mohits

lib/tasks/turbo_tasks.rake

@@ -3,7 +3,9 @@ def run_turbo_install_template(path)
 end
 
 def redis_installed?
-  system('which redis-server > /dev/null')
+  Gem.win_platform? ?
+    system('where redis-server > NUL 2>&1') : 
+    system('which redis-server > /dev/null')
 end
 
 def switch_on_redis_if_available