Bump the minimum supported version of WordPress to 3.5

Currently, we guarantee that _all_ subcommands work with WP 3.4 or newer. The latest WP version is 3.8.1.

We are starting to see issues that only affect WP 3.4:

* `wp theme install --activate` (see #875; works fine in WP 3.5)
* `wp_insert_user()` doesn't accept an object (fixed in WP 3.5: https://core.trac.wordpress.org/ticket/21429)

Note that old WP installs should still be upgradable using the `wp core download --force` command.