use frozensets in apriori

[rasbt]

Description

Use frozensets instead of regular sets in mlxtend.frequent_patterns.apriori to improve selection efficiency

Related issues or pull requests

Just a minor improvement after #393

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran nosetests ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., nosetests ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

[coveralls]

Coverage increased (+0.04%) to 91.308% when pulling 203951f on frozenset into 84fde97 on master.