Update Gemfile

Remove github link to personal fork
Keeping personal master working until pull requests are taken
Changes Gemfile.lock to reflect latest personal ruby-sdl-ffi github version
Add button_up events

Modified logic to send event on button change, selecting button_* or
button_up_* depending on state.  Previous button events are only sent
on down event, and only button_up_* events are sent when the button is
released.
Adding to available commands to avoid warning messages
Add currently_pressed? command

currently_pressed? takes either a symbol or index value to return that
button's current state.
Updated gem versions
Adding currently_pressed? command and button_up events