The option is still there in the same place, it's just when you click on the swatch options and go to colour mode RGB looks as though it's sitting at the top of the pallette, however you just need to scroll up to reveal the other colour options
Oh yeah!! Thank you, thank you!
When you select an existing swatch or click create new swatch a Swatch Options menu opens. In the Color Mode pull down window you have multiple choices including CMYK and RGB.