add some linting to CI

[oconnor663]

@TalhaKhatri fixed a bunch of Clippy warnings in #52. It would be nice to get something into Travis to make sure we don't regress these fixes over time.

One thing to think about: I get the impression that Clippy adds new lints all the time. That could mean that our code "breaks" without actually changing, just because Clippy started checking something new. Maybe the standard way to deal with this is to pin a specific version of Clippy? I haven't looked into it yet.

[BrainMaestro]

Clippy adds new lints, but usually they don't make new lints errors unless for a very good reason. See the categories. I think the default might be okay, and just customize if necessary