Use references to store constants::GetValues().

Most of the code already uses auto references so this should make it
consistent.

My very authoritative source of information regarding how auto works:
http://stackoverflow.com/questions/8542873/c11-auto-semantics

Change-Id: I0ca437c564a5f878e1c716fd858770bbdf821687
diff --git a/frc971/wpilib/wpilib_interface.cc b/frc971/wpilib/wpilib_interface.cc
index 1941144..c60feec 100644
--- a/frc971/wpilib/wpilib_interface.cc
+++ b/frc971/wpilib/wpilib_interface.cc
@@ -254,7 +254,7 @@
 
     dma_synchronizer_->RunIteration();
 
-    const auto values = constants::GetValues();
+    const auto &values = constants::GetValues();
 
     {
       auto fridge_message = fridge_queue.position.MakeMessage();