I vote for buy a second bike (says the person with 11 bikes in a 2 person home).
Why?
Commute miles can be hard on a bike. You'll have to do a lot of fiddling to get things just right on your Ruby and you'll be making her do things she wasn't really designed for.
Some changes will be hard to take off for a fast weekend ride.
No matter what you do, your carrying capacity/tire size is limited. I'd recommend 32's for the commute you describe, and I bet you can't fit more than a 25 or 28 on your Ruby.
Theft - how secure is your workplace.
New bikes are fun
Lots of good commute options:
Surly LHT or Cross Check (steel, take racks, take wide tires, can be found used)
Folding Bike (Bike Friday, Dahon, etc. - lots of options, a wide variety of prices - may be able to come inside with you and gets you a travel bike option).
Jamis - some nice touring bike options
Depending on the length of your commute - maybe a mountain bike/hybrid?
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