dev: Add policy for updating the minimum version of our dependencies

Could be inspired by Matplotlib's:
https://matplotlib.org/devel/min_dep_policy.html

Note that we currently don't specify the minimum versions explicitly. For example, we currently implicitly support Python 3.4 by running Travis tests that use Python 3.4. See the f-strings commit in this PR:
https://github.com/brainiak/brainiak/pull/384#commits-pushed-dc297fc