Add joblib to tests' dependencies

[StrikerRUS]

Related #542.

With recently added test

m2cgen/tests/test_cli.py

Lines 127 to 131 in b0f76ef

	def test_joblib_loading(pickled_model):
	mock_args = _get_mock_args(infile=pickled_model, language="go", lib="joblib")
	generated_code = cli.generate_code(mock_args).strip()
	assert generated_code.startswith("func score(input []float64) float64 {\n")

we need joblib library installed during running API tests.

I believe it's better to install this library explicitly rather than relying on indirect installation of joblib along with scikit-learn.