switch to Codecov for coverage reporting

README.rst

@@ -425,3 +425,5 @@ file in the top distribution directory for the full license text.
     :alt: Bitdeli badge
     :target: https://bitdeli.com/free
 
+.. |coverage-status| image:: https://codecov.io/gh/celery/celery/badge.svg
+   :target: https://codecov.io/gh/celery/celery

requirements/test-ci.txt

@@ -1,3 +1,3 @@
 coverage>=3.0
-coveralls
+codecov
 -r extras/redis.txt