Upgrade axe-core in addon

[Melanie Sumner]

• upgrade axe-core version in addon
• analyze potential for something automated (feels like this should be possible, but idk yet).

[Trent Willis]

The first part of this seems done now.

analyze potential for something automated

There's greenkeeper, but I don't believe it works for Bower dependencies. That said, we should get off of using Bower and move instead to including it as an npm module.

[Melanie Sumner]

@trentmwillis since that's a separate issue, I'm going to close this issue. :)