Update the Appveyor Status Badge.

Currently the status badge points to all the builds triggered in the Appveyor including the Pull Request builds. So if a PR build fails the build status on readme.md will show failure. This change will make the status badge in readme.md to point to the latest build on the dotnet/corefx master branch (excluding the PR builds).
Merge pull request #132 from Sridhar-MS/appveyor-badge-update

Update the Appveyor Status Badge.