Change coverage threshold for failing build status (#561)

Summary:
See title

Pull Request resolved: flashlight/flashlight#561

Test Plan: CI

Reviewed By: tlikhomanenko

Differential Revision: D27966313

Pulled By: jacobkahn

fbshipit-source-id: d8bcc90437b170ba56b0043809e753db51b257e4

codecov.yml

@@ -1 +1,6 @@
 comment: false
+coverage:
+  status:
+    project:
+      default:
+        threshold: 0.25%