Fail Travis CI build on eslint warnings

[kevinschaper]

Since they're so easy to miss while developing, and we don't want them to get out of hand, we might as well get our CI process involved.

Over in slack, @sibyl229 found this helpful issue that talks about how to configure for error exit codes (and some more advanced tweaks to show all errors, but maybe we don't need that?)

And @ragingsquirrel3 mentioned that there's a separate webpack config for test, so we can get the lint output to start showing up by editing that.

[ragingsquirrel3]

I think you take https://github.com/alliance-genome/agr/blob/development/webpack.config.babel.js#L36 and put it in https://github.com/alliance-genome/agr/blob/development/webpack.config-test.js