.travis.yml: Use Python version '3.6'
.travis.yml: Use Python version '3.6'

Python 3.6 has been released, and the old Travis CI '3.6-dev'
environment can be accessed using '3.6'.
RonnyPfannschmidt/setuptools_scm,pypa/setuptools_scm,RonnyPfannschmidt/setuptools_scm,pypa/setuptools_scm