README: Point to libgit2 account's AppVeyor badge

Microsoft is sponsoring a Pro account at AppVeyor for the libgit2
and LibGit2Sharp projects.  Point to that account's badge.

README.md

@@ -2,7 +2,7 @@ libgit2 - the Git linkable library
 ==================================
 
 [![Travis Build Status](https://secure.travis-ci.org/libgit2/libgit2.svg?branch=master)](http://travis-ci.org/libgit2/libgit2)
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 `libgit2` is a portable, pure C implementation of the Git core methods