Bump dependencies

[mcdenhoed]

Ah, that's a good point. Yeah, we should remove it. If we're blocking merges based on rustfmt, I see no reason to have a separate stable build without it.

[mcdenhoed]

Also, is there a particular reason to do a beta build? Could probably just do a single stable build with rustfmt and a single nightly with clippy.

[johnthagen]

Beta build is mostly to help the Rust community. The idea is that builds should never break from stable to beta, and if they do it is a reportable issue/bug. This has actually happened before on rust-belt early on. In those cases it was real problems with an upstream crate and/or the compiler. So it's fun to actually have a chance to contribute back in that way. See this link.

The Rust team appreciates testing against the beta and nightly channels, even if you are only targeting stable

For me the ideal builds I would like to use are:

• Stable Linux
• Stable macOS
• Beta Linux
• Nightly Linux
• Stable Linux rustfmt
• Nightly Linux clippy

Justification for this configuration:

• Linux builds are "cheap" for Travis and rust-belt doesn't have that many builds
• Builds run (generally, other than macOS in peek load) in parallel, so more builds doesn't slow down overall time to completion
• I like separating rustfmt and clippy into their own builds because if a build fails, it's immediately obvious why it failed based on the build type. If the rustfmt build fails, I know I don't need to even look at the build log, I just need to run the latest rustfmt locally and push.

If you'd like, we can merge your PR as is (it passes rustfmt and bumps the dependancies), and I'll open a separate PR to reconfigure Travis.