I am using RubyMine as well and when I copy pasted your code, I didn't see any problem.
I always use 'watir-webdriver', which interanlly uses selenium webdriver (supports different browsers) instead of 'watir'.
Goto Tools -> Show Gem Environment and check whether you have 'watir-webdriver' listed.
If it is not listed there then goto Tools -> Terminal and install the watir-webdriver gem.
Refresh the project or restart the IDE to see the changes.
Hope it helps for you.