travis: move away virtualenv's distutils when running Pylint.
travis: move away virtualenv's distutils when running Pylint.

Fixes:

    setup.py:172: E: no-member [cmd_install_doc.run] Instance of 'cmd_install_doc' has no 'outfiles' member
    setup.py:174: E: no-member [cmd_install_doc.run] Instance of 'cmd_install_doc' has no 'outfiles' member

Work-around for: https://github.com/PyCQA/pylint/issues/73

jwilk/pydiatra,jwilk/pydiatra