Check wp-includes/version.php before loading WP

Currently, if you try to run any subcommand on a WP install that's older than 3.4, you get no output, because error reporting is disabled by default. That's very confusing, especially to new users.

Before begging the loading process, we could `include_once wp-includes/version.php;` and throw an error if the version doesn't meet the minimum requirements.