Fixed drivetrain code building issue.

Change-Id: I630d61a2122956f62534142c37e8282efc8cdb0c
diff --git a/frc971/control_loops/drivetrain/polydrivetrain.cc b/frc971/control_loops/drivetrain/polydrivetrain.cc
index cefb1dd..9dad2a8 100644
--- a/frc971/control_loops/drivetrain/polydrivetrain.cc
+++ b/frc971/control_loops/drivetrain/polydrivetrain.cc
@@ -68,6 +68,7 @@
       return high_gear_speed;
     case LOW:
     case SHIFTING_DOWN:
+    default:
       return low_gear_speed;
       break;
   }