Auto merge of #5901 - dwijnand:1.28-win, r=alexcrichton
Auto merge of #5901 - dwijnand:1.28-win, r=alexcrichton

Align AppVeyor to use 1.28 for minimal versions

.. like Travis CI's setup.

eminence/cargo,vojtechkral/cargo,vojtechkral/cargo,rust-lang/cargo,tsurai/cargo,eminence/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,tshepang/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,tshepang/cargo,tsurai/cargo,rust-lang/cargo,rustyhorde/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,rustyhorde/cargo,vojtechkral/cargo,tsurai/cargo,nipunn1313/cargo,nipunn1313/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo