Unfortunately the 15-85 (quite good), 28-135 (excellent value used) and 18-135 (good, convenient range) all would merely replace your 18-55 with another "walk around" lens that doesn't have particularly large aperture, so isn't all that usable in low light. Yes, they all would be an improvement over the basic kit lens and all have IS which can help steady your shots... But IS can't help stop subject movement... For that you need a larger aperture and a higher shutter speed. All the zooms being discussed and most in this general price range are even slower f3.5-5.6. One possibility... the Tamron 17-50/2.8 non-IS is under $500 the last time I looked. It's a sharp lens, but it's AF isn't as fast and accurate as some other options. And, it's "only" f2.8. There are no zooms available for Canon that are any faster than f2.8 (and most of those are quite pricey).
I think one or more faster (larger aperture) primes are going to be your best choices...
The Sigma 30/1.4 is a good suggestion. It's two full stops faster than any f2.8 zoom, up to four stops faster than some on the list above. It would complement your 50/1.8 nicely (but might spoil you... it's a much better made, better focusing lens in many situations... next you might want to upgrade your 50mm!). However, the 30/1.4 is known to not be particularly good at focusing in low light, such as in a bar. And, it can be somewhat of a lottery getting a "good copy" of the lens (which is probably why they are on sale now... I bet Sigma is blowing them out and planning to replace with their newer generation lenses that are tunable by the user and held to higher calibration standards).
The Canon 28/1.8 is a possible alternative. It's 2/3 stop slower than the Sigma, but has quality USM focus, is capable of very nice images, and is better built than your 50/1.8 (again, it's going to make you want to upgrade the 50mm). As an EF lens, the Canon is also usable on both crop and full frame, too... should that ever become necessary. The Siggy 30/1.4 is crop only. Even with its matched lens hood, the Canon 28/1.8 also is relatively compact for it's series (i.e., Canon "mid-grade/USM".... a couple steps up from the entry level 50/1.8, but not premium L-series).
The 85/1.8 is similar in build/performance to the 28/1.8 (except that it's a bit larger, as is the 50/1.4). But, 85mm is a "long portrait" tele on a crop sensor camera, where 50mm is a "short portrait" tele and 28mm is a "couple, small group, full length portrait" normal lens. Eventually you might want to add an 85mm too... That's the nature of prime lenses... One is never enough, two is better, three or more is even better still!
Sorry, but primes are always going to be "less versatile" than zooms... You just have to learn to "zoom with your feet".
Wanting to shoot in a bar, if you don't already have one you might want to plan to get an accessory flash, too. Put it on a flash bracket, connected to the camera with an off-camera shoe cord, for better quality shots. Most cameras have built in flashes, but they are anemic, in the worst possible place for redeye and ugly shadows, and they rely upon and drain the camera's battery(ies) pretty rapidly.
Another challenging thing about shooting in a bar is focusing in low light, even with the best lenses and camera systems. Short of a 5D Mark III or 1DX, which are rated to be able to focus at -3 EV, far lower than most previous models, there are other things you can do. Many accessory flashes are capable of emitting a Focus Assist in the form of a near-IR grid that helps the camera and lens focus. (And, yes, the cameras with a built in flash can emit a Focus Assist of sorts, too... But it's an obnoxious series of white light flashes, that drain the battery, can overheat the flash if used a lot, and might make you rather unpopular in bars.) If you don't want to work with flash, you can use only the AF Assist feature, setting the camera to not fire the flash. Or, you can use a transmitter module like the ST-E2 or ST-E3, which is also capable of emitting a near-IR Focus Assist, but is much smaller than a flash.
