Fixed deprecated warning for constructor, Updated README

Got tired of seeing these appear in my Developer Logs.

`Deprecated function EE_Fieldtype() called in [...]/wb_category_select/ft.wb_category_select.php on line 27.
Deprecated since 2.6. Use EE_Fieldtype::__construct() instead.`

Seemed simple enough to fix, left this on a development site for a number of days and have not seen the warning pop up again. (Let me know what the protocol is for changing the README, version #, etc. It seemed someone else changed it to 1.3.5 but the README wasn't updated along with, so figured why not.)