Updated Dockerfile for 15.10 to 3.9.
Updated Dockerfile for 15.10 to 3.9.

[ci skip]
jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius