appveyor: simplify build script
appveyor: simplify build script

This removes the matrix configuration which allowed breakage on
nightly -- I don't think I've ever seen nightly Rust break my crate.
Even if it did, I would like to know about it and try and fix it.

Caching is removed to prevent accidentally hiding problems when our
dependencies release new versions.

Finally, the --verbose flag adds precise version information to the
log files. This has not been useful so far, so I'm removing it.

mgeisler/textwrap