Add sacrebleu and rouge-score as required dependencies

[jaalu]

Cloning the project, installing dependencies with pip install -r requirements.txt and running tests with python3 -m pytest currently fails since sacrebleu and rouge-score are not explicitly listed as dependencies:

ImportError while importing test module '/home/jlu/comparestrings/string2string/tests/test_rogue.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_rogue.py:7: in <module>
    from string2string.metrics import ROUGE
string2string/metrics/__init__.py:3: in <module>
    from .sbleu import sacreBLEU
string2string/metrics/sbleu.py:7: in <module>
    from sacrebleu import corpus_bleu
E   ModuleNotFoundError: No module named 'sacrebleu'

This PR adds the packages in question as dependencies in setup.py and requirements.txt, fixing the tests and solving #4.

Questions:

• Should the version number in setup.py be updated?
• I've also added requirements-dev.txt with frozen package versions where the tests are known to run - which could potentially make it easier to debug breakages due to backwards-incompatible changes in dependencies. Should this file be renamed or included at all?