ENSCORESW-3148: update Coveralls URIs in the README

The Coveralls URI need to contain the branch name for the badge to work,
moreover URIs for GitHub repositories are (now?) different. The URIs use
now both are what Coveralls Web site suggests and have been demonstrated
to work in case of ensembl-xref.

README.md

 # Ensembl Core API
 
 [![Build Status](https://travis-ci.org/Ensembl/ensembl.png?branch=master)][travis]
-[![Coverage Status](https://coveralls.io/repos/Ensembl/ensembl/badge.png)][coveralls]
+[![Coverage Status](https://coveralls.io/repos/github/Ensembl/ensembl/badge.svg?branch=master)][coveralls]
 
 [travis]: https://travis-ci.org/Ensembl/ensembl
-[coveralls]: https://coveralls.io/r/Ensembl/ensembl
+[coveralls]: https://coveralls.io/github/Ensembl/ensembl