Display stdlib errors on Travis run

[systemcatch]

So I figured out how we can display stdlib errors on a Travis run: we can run nose as a module and set a flag (-Wd)for Python to display all warnings.

I was thinking we could make a separate target for testing the dev releases:

test-dev:
	rm -f .coverage
	. local/bin/activate && local/bin/python3 -Wd -m nose

Originally posted by @jadchaar in #585 (comment)

[jadchaar]

Gonna try and tackle this when I get some free time :)

[jadchaar]

I added this to the Makefile in the latest PR so it can be run before future Python releases going forward. I am not putting it in Travis because the only warnings emitted are by the dated nose and by a deprecation warning emitted by arrow itself. I think adding it to Travis could introduce extra clutter and confusion :).