DOC: update URL for codecov badge

Author: pavoljuhas

README.rst

@@ -1,8 +1,8 @@
 .. image:: https://travis-ci.org/diffpy/diffpy.srreal.svg?branch=master
    :target: https://travis-ci.org/diffpy/diffpy.srreal
 
-.. image:: http://codecov.io/github/diffpy/diffpy.srreal/coverage.svg?branch=master
-   :target: http://codecov.io/github/diffpy/diffpy.srreal?branch=master
+.. image:: https://codecov.io/gh/diffpy/diffpy.srreal/branch/master/graph/badge.svg
+   :target: https://codecov.io/gh/diffpy/diffpy.srreal
 
 diffpy.srreal
 ========================================================================