This PR upgrades Gridsync's PyQt5 requirement to version 5.12 but also changes various build-related scripts/configs to downgrade PyQt5 to a known working/compatible version in the event that the host is running an older operating system (including, specifically, Debian 8, Ubuntu 14.04, and macOS 10.11 -- all of which are now unsupported upstream). In addition, tox.ini and the Makefile have been cleaned up somewhat to make platform-specific builds less convoluted; users can now simply run make to get a working PyInstaller-generated binary using the last-supported Qt5 version for that system (instead of, specifying, e.g., make macos-legacy for older versions of macOS). As another convenience, make test will now run the test suite, conditionally wrapping the tox call under xvfb-run on non-macOS *nix systems (rather than dealing with tox.inis fairly limiting "platform" tags to achieve the same result).