Bump rake-compiler and associated deps

Also regenerate Gemfile for usage with rake-compiler-dev-box
Introduce AppVeyor CI for Windows

The included configuration aims to compile and test builds of
sqlite3-ruby gem against a native Windows environment provided by
AppVeyor platform.

Add `devkit` task that prepends both compile and native ones and
ensures RubyInstaller's DevKit is enabled in the PATH prior
compilation.