try travis 3.7

[rasbt]

Description

In this particular Python 3.7 miniconda environment, pip has trouble installing coveralls

pip._vendor.pkg_resources.RequirementParseError: Invalid requirement, parse error at "'; extra '"

Hence, for now, just use Python 3.6 for coveralls

Related issues or pull requests

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