Adjust codecov config (codecov.yml) for commit status (#754)

This fix adjusted codecov config (codecov.yml) so that
commit status is success as long as coverage > 40%.

This fix fixes #752.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

codecov.yml

+coverage:
+  status:
+    project:
+      default:
+        target: 40%
+        threshold: null