Problem: Codecov shield commented-out in README.md (#2421)

Solution: Uncomment it, since Codecov is working again

![Codecov](https://img.shields.io/codecov/c/github/bigchaindb/bigchaindb/master.svg)

README.md

@@ -2,9 +2,8 @@
 (including pre-release versions) from PyPI,
 so show the latest GitHub release instead.
 --->
-<!--- Codecov isn't working for us lately, so comment it out for now:
+
 [![Codecov branch](https://img.shields.io/codecov/c/github/bigchaindb/bigchaindb/master.svg)](https://codecov.io/github/bigchaindb/bigchaindb?branch=master)
---->
 [![Latest release](https://img.shields.io/github/release/bigchaindb/bigchaindb/all.svg)](https://github.com/bigchaindb/bigchaindb/releases)
 [![Status on PyPI](https://img.shields.io/pypi/status/bigchaindb.svg)](https://pypi.org/project/BigchainDB/)
 [![Travis branch](https://img.shields.io/travis/bigchaindb/bigchaindb/master.svg)](https://travis-ci.org/bigchaindb/bigchaindb)