Update coverage status badge image

coveralls.io is currently down, causing the coverage status badge to show as a broken image.  This replaces the badge link with one that appears to be reliable in the face of the service being down, showing a "currently unavailable" badge instead of a broken link.

README.md

@@ -4,7 +4,7 @@
 |:-:|:-:|:-:|:-:|
 |**Debug**|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_linux_debug/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_linux_debug/)|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_windows_debug/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_windows_debug/)|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_mac_debug/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_mac_debug/)|
 |**Release**|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_linux_release/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_linux_release/)|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_windows_release/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_windows_release/)|[![Build status](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_mac_release/badge/icon)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_mac_release/)|
-|**Coverage Report**||[![Coverage Status](https://coveralls.io/repos/dotnet/corefx/badge.svg?branch=master)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_coverage_windows/lastStableBuild/Code_Coverage_Report/)||
+|**Coverage Report**||[![Coverage Status](https://img.shields.io/coveralls/dotnet/corefx/master.svg)](http://dotnet-ci.cloudapp.net/job/dotnet_corefx_coverage_windows/lastStableBuild/Code_Coverage_Report/)||
 
 The corefx repo contains the library implementation (called "CoreFX") for [.NET Core](http://github.com/dotnet/core). It includes System.Collections, System.IO, System.Xml and many other components. It builds and runs on Windows. You can ['watch'](https://github.com/dotnet/corefx/subscription) the repo to see Linux and Mac support being added over the next few months.