bumping version
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.
Alter requisites for `compile` and `native` on Windows

Previous definition caused issues when compiling natively since
DevKit was not enabled at the proper time.

This is caused by ports getting prepend into the dependency list
of `compile` task way before than it should have been.

This change, while feels like a hack, gets the job done.

And I'm making a cup of tea to celebrate it :smile:
Remove `--enable-local` option

This has been useless since the removal of `recipe.active` and
the tweaks of `LDFLAGS` are no longer necessary when using extconf's
options.
Use local build when running on MinGW

Since LDFLAGS are not updated (see previous commit) we now need to
use `--with-opt-dir` option to ensure local install of SQLite3 is
used.
Properly handle both `compile` and `native gem` tasks

AppVeyor CI was setup to run `native gem` task, but neither DevKit
or port was properly hook up for it to work.

This change aims to correct that.
Update SQLite reference to latest

Native version to be used has been bumped to 3.8.6