Switch genesis_get_additional_image_sizes for genesis_get_image_sizes

[nickcernis]

(In the feature/portfolio-widget branch.)

• genesis_get_additional_image_sizes is deprecated
• genesis_get_image_sizes returns more image options

Have self-assigned for 1.2.

[nickcernis]

Function swapped. All image sizes are now available to choose from:

And no more deprecation warnings.

Closing here for review as part of https://github.com/studiopress/genesis-portfolio-pro/pull/24.