Increase codecov target requirement to 50% (from 40%) (#1344)

It looks like our code coverage is above 50% now. Let's
make it a requirement to prevent it from regress.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.codecov.yml

@@ -2,7 +2,7 @@ coverage:
   status:
     project:
       default:
-        target: 40%
+        target: 50%
         threshold: null
     patch: false
     changes: false