Remove Coveralls configuration and update README to reflect changes (#4777)

Author: RishabhJain2018

.coveralls.yml

@@ -5,7 +5,6 @@
 [![Join the chat on Slack](https://img.shields.io/badge/Join%20Slack-Chat-blue?logo=slack)](https://join.slack.com/t/cloudcv-community/shared_invite/zt-3252n6or8-e0QuZKIZFLB0zXtQ6XgxfA)
 [![Build Status](https://travis-ci.org/Cloud-CV/EvalAI.svg?branch=master)](https://travis-ci.org/Cloud-CV/EvalAI)
 [![codecov](https://codecov.io/gh/Cloud-CV/EvalAI/branch/master/graph/badge.svg)](https://codecov.io/gh/Cloud-CV/EvalAI)
-[![Coverage Status](https://coveralls.io/repos/github/Cloud-CV/EvalAI/badge.svg)](https://coveralls.io/github/Cloud-CV/EvalAI)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Documentation Status](https://readthedocs.org/projects/markdown-guide/badge/?version=latest)](http://evalai.readthedocs.io/en/latest/)
 [![GitHub commit activity](https://img.shields.io/github/commit-activity/m/Cloud-CV/EvalAI?style=flat-square)](https://github.com/Cloud-CV/EvalAI/tree/master)