Update AppVeyor build status badge URL.
Update AppVeyor build status badge URL.

Since we set up AppVeyor project again, the build status badge URL
was changed.

kbc-developers/Mozc,kbc-developers/Mozc,kbc-developers/Mozc,kbc-developers/Mozc,kbc-developers/Mozc,kbc-developers/Mozc,kbc-developers/Mozc