gather_builder_stats: fix the sheets version

Decrement the sheets version by one so we don't overwrite previous
rows. This is necessary because old metadata.json file does not have
the new 'reason' field.

BUG=None
TEST=None

Change-Id: Ifcdff90fb6475e59a82457eba26e606c9fdf103a
Reviewed-on: https://chromium-review.googlesource.com/198130
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
diff --git a/scripts/gather_builder_stats.py b/scripts/gather_builder_stats.py
index d53971d..021e938 100755
--- a/scripts/gather_builder_stats.py
+++ b/scripts/gather_builder_stats.py
@@ -128,7 +128,7 @@
   """Stats table for master builder that puts results in a spreadsheet."""
   # Bump this number whenever this class adds new data columns, or changes
   # the values of existing data columns.
-  SHEETS_VERSION = 3
+  SHEETS_VERSION = 2
 
   # These must match up with the column names on the spreadsheet.
   COL_BUILD_NUMBER = 'build number'