Use different install-board command line switch for windows

Author: ianfixes

lib/arduino_ci/arduino_cmd_windows.rb

@@ -9,7 +9,7 @@ class ArduinoCmdWindows < ArduinoCmd
     flag :set_pref,        "--pref"
     flag :save_prefs,      "--save-prefs"
     flag :use_board,       "--board"
-    flag :install_boards,  "--install-boards"
+    flag :install_boards,  "--install-board"
     flag :install_library, "--install-library"
     flag :verify,          "--verify"
   end