Update Backbone to 1.4.0 and Underscore to 1.9.1

This updates Backbone from version 1.1.2 to 1.4.0 and Underscore from
1.7.0 to 1.9.1. These versions should be compatible, but you can check
out the changelogs for [Backbone][1] or [Underscore][2] if you're
curious.

I tested the site manually and it appears to work just fine after this
change.

Partially addresses [issue #2][3].

[1]: https://backbonejs.org/#changelog
[2]: https://underscorejs.org/#changelog
[3]: #2