ci: Add NetBSD target to build on Travis CI

[iquiw]

As discussed in #1594 (EDIT: wrong issue referred, sorry)

Travis build succeeded.
https://travis-ci.org/iquiw/rustup.rs/jobs/579416112

[iquiw]

hmm, i686-unknown-linux-gnu failed.

+ cargo build --locked -v --release --target i686-unknown-linux-gnu --features vendored-openssl
    Updating crates.io index
error: failed to download from `https://crates.io/api/v1/crates/git-testament/0.1.6/download`
Caused by:
  [35] SSL connect error (error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure)

[bors]

☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

[kinnison]

We don't target 'stable' for merges, we target 'master' so you need to rebase your change to work on master. Once that's done, I will merge it because it looks good.

[iquiw]

Rebased, but one job failed.
https://travis-ci.com/rust-lang/rustup.rs/jobs/230083993

[tesuji]

Do not worry about it. The beta clippy is regressing.

[kinnison]

@iquiw Thank you for your efforts. I'm going to merge this and we can keep an eye on it in master.