Revert "add a codecov badge to the README"

This reverts commit ecf8ed4.

codecov is useless and it's a nuisance. We don't use tests/
directory or testit package and codecov gives misleading and
annoying reports.

README.md

@@ -4,4 +4,4 @@
 [![CRAN version](http://www.r-pkg.org/badges/version/vegan)](http://cran.rstudio.com/web/packages/vegan/index.html) [![](http://cranlogs.r-pkg.org/badges/grand-total/vegan)](http://cran.rstudio.com/web/packages/vegan/index.html)
 
 #### Build status
-[![Build Status](https://travis-ci.org/vegandevs/vegan.svg?branch=master)](https://travis-ci.org/vegandevs/vegan)  [![Build status](https://ci.appveyor.com/api/projects/status/hc8dbxrim2nj3c1i/branch/master)](https://ci.appveyor.com/project/vegandevs/vegan/branch/master) [![codecov.io](https://codecov.io/github/vegandevs/vegan/coverage.svg?branch=master)](https://codecov.io/github/vegandevs/vegan?branch=master)
+[![Build Status](https://travis-ci.org/vegandevs/vegan.svg?branch=master)](https://travis-ci.org/vegandevs/vegan)  [![Build status](https://ci.appveyor.com/api/projects/status/hc8dbxrim2nj3c1i/branch/master)](https://ci.appveyor.com/project/vegandevs/vegan/branch/master)