Downgrade NetBSD's Rust version

[mattico]

to workaround a build failure
rust-lang/rust#49627

[Michael-F-Bryan]

I was actually thinking we should cut out a lot of CI builds and go back to running on the raw VM.

Due to the way cross is set up, we essentially have to re-download and re-compile everything every time a build is run, I don't believe it lets you use a cached target/ directory. This means each build takes upwards of 20 minutes, even if there was a trivial change (e.g. updated styles or README).

The targets I'd go for are 32-bit and 64-bit Linux, plus an OS X build. It means we have less pre-compiled binaries available from GitHub Releases, but it should reduce a lot of the pressure on travis and speed up the turnaround time.

Thoughts?

[mattico]

That sounds good to me. We're a pretty simple project. If something doesn't work, it's probably a bug in rustc or a dependency.

[Michael-F-Bryan]

@mattico I'm going to close this PR in favor of #664.