CI: Bump warnings up to errors in an allowed-failure mode

[effigies]

To stay on top of pending deprecations and to not become one of those libraries that causes all downstream packages to emit tons of warnings or fiddle with warnings filters, we should set some Travis jobs to turn warnings into errors. To allow this to be opportunistic, rather than a constant pain, we should mark these jobs as allow_failure.

It looks like Pytest has some mechanisms for modifying warning behavior, which will probably work better than the PYTHONWARNINGS="error" environment variable tried out in #84.