Install pandas, so bench/run_benchmarks.sh runs from a clean install

@changtimwu hit this in #22: every cohort dies on "Please install vllm[bench]
for bench support", because vllm/benchmarks/datasets/datasets.py imports pandas
through a PlaceholderModule and the message names the extra rather than the
package. Installing vllm[bench] is the wrong fix -- it reinstalls vLLM and
reverts the whole patch stack.

Only pandas is needed. The extra's other members are for HF-hosted
--dataset-name values and the plotting subcommands, and run_benchmarks.sh uses
only `random` and `custom`; the reference box has no `datasets` installed and
runs the full suite.