The terms of the site - CC-by-SA - require you to attribute your source, and - if you were to make more of these T-shirts - only offer it under similar terms.
I've never heard anything formal or official about clothing and CC-by-SA text, so I'm willing to bet that you couldn't take the text, make the T-shirt and profit from it. A private shirt, though - I don't think anyone's going to come after you.
EDIT: Looking back on the full legal text of CC-by-SA 3.0 Unported, section 3(a) jumps out at me.
3. License Grant. Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive,
perpetual (for the duration of the applicable copyright) license to
exercise the rights in the Work as stated below:
a) to Reproduce the Work, to incorporate the Work into one or more
Collections, and to Reproduce the Work as incorporated in the
A t-shirt would be considered a reproduction, the end result would be royalty-free (meaning, as best as I can tell that you wouldn't owe StackExchange money), and the license would allow someone else to use the same work in the same manner (by CC-by-SA).
I think you can do it. Place the CC-by-SA tag somewhere near the bottom/back, and/or place a QR code to the full license (finally a use for QR codes!) to satisfy 4(a).