Switch to 6 joystick axis now that we are using wpilib 2017

Change-Id: I7ad94990952c932ecf00fd3d82b59d72ba31ebcf
diff --git a/aos/common/messages/robot_state.q b/aos/common/messages/robot_state.q
index 3ecb653..1e9b936 100644
--- a/aos/common/messages/robot_state.q
+++ b/aos/common/messages/robot_state.q
@@ -4,8 +4,8 @@
   // A bitmask of the button state.
   uint16_t buttons;
 
-  // The 4 joystick axes.
-  double[4] axis;
+  // The 6 joystick axes.
+  double[6] axis;
 
   // The POV axis.
   int32_t pov;