[stable-2.7] Fix run.py --branch argument.

The API docs state that both `branch` and `branchName` are valid,
but only `branchName` appears to work.
(cherry picked from commit 31c1bdd6a8)

Co-authored-by: Matt Clay <matt@mystile.com>

test/utils/shippable/tools/run.py

@@ -103,7 +103,7 @@ def main():
     )
 
     if args.branch:
-        data['branch'] = args.branch
+        data['branchName'] = args.branch
     elif args.run:
         data['runId'] = args.run