Require later version of transformers

[tleyden]

Currently it doesn't require any particular version of transformers, however running evo merge with v4.35.0 gives this error:

ray.exceptions.RayTaskError(TypeError): ray::InMemoryMergeEvaluator.evaluate_genotype() (pid=1424, ip=172.17.0.4, actor_id=7fc68474338a238d957914ee01000000, repr=<mergekit.evo.actors.InMemoryMergeEvaluator object at 0x7ff0846eb610>)
  File "/mergekit/mergekit/evo/actors.py", line 303, in evaluate_genotype
    return self.evaluate(genotype)
  File "/mergekit/mergekit/evo/actors.py", line 236, in evaluate
    self._maybe_init_model(config)
  File "/mergekit/mergekit/evo/actors.py", line 182, in _maybe_init_model
    transformers.AutoModelForCausalLM.from_config(
  File "/opt/conda/lib/python3.10/site-packages/transformers/models/auto/auto_factory.py", line 441, in from_config
    return model_class._from_config(config, **kwargs)
  File "/opt/conda/lib/python3.10/site-packages/transformers/modeling_utils.py", line 1192, in _from_config
    model = cls(config, **kwargs)
TypeError: MistralForCausalLM.__init__() got an unexpected keyword argument 'attn_implementation'

WDYT of requiring transformers>=4.39.3, which is the latest version used by the HF Alignment Handbook and should probably work (untested)

[cg123]

Done in #331.