Pinned Crossbeam's version in an attempt to address #8

[joshmarlow]

Purpose

There appears to be a breaking API change in crossbeams which duct depends on.
Building locally works correct, however, cargo install cargo-test-junit fails as indicated in #8.
This PR attempts to address this build issue.

What appears to be the cause of the failure

When building locally, Cargo.lock - which pins an older version of crossbeams - is respected.
However, according to [0], when uploading the package to crates.io, no Cargo.lock is uploaded.

I've confirmed that checking out cargo-test-junit locally and running cargo build works correctly.
However, updating Cargo.lock first (ie, cargo update; cargo build) results in the same build error as when installing.

Upgrading duct results in different build failures due to changes in the duct API. Pending someone taking the effort to upgrade cargo-test-junit to use the newer version of duct, it seems reasonable to pin the version of crossbeam in cart-test-junit's Cargo.toml file.

That being said, I do not know how to test this more thoroughly than pushing an updated package to crates.io.

[0] - https://users.rust-lang.org/t/cargo-install-and-lock-file/4203