Change minimum ICU version to 4.6
Change minimum ICU version to 4.6

Thanks to Brandon Kraft's comment,
it looks like this version has teh
INTL_IDNA_VARIANTE_UTS46 constant:
https://github.com/ampproject/amp-wp/issues/1439#issuecomment-422074851
And as long as that constant is defined, it will be passed to idn_to_utf8():
https://github.com/ampproject/amp-wp/blob/39336b78be7a4ba8b3a09a6f43d9f66e3c201e05/includes/class-amp-http.php#L233
...and that won't trigger this deprecation warning:
https://github.com/php/php-src/pull/2355/files#diff-24931a05f9e656989a55c44cf8a72f9eR294

ampproject/amp-toolbox-php,ampproject/amp-toolbox-php