clipboard not compiling on Travis CI linux targets

[Ninjani]

Thanks for this crate, I'm using it for a terminal code snippets manager here. I'm having some issues building binaries for Linux using Travis, and the failure seems to be related to clipboard's xcb dependency. This is the error:

error: failed to run custom build command for `xcb v0.8.2`

Caused by:

  process didn't exit successfully: `/target/debug/build/xcb-642392b748273c1b/build-script-build` (exit code: 101)

--- stderr

thread 'main' panicked at 'Unable to find build dependency python3: Os { code: 2, kind: NotFound, message: "No such file or directory" }', /cargo/registry/src/github.com-1ecc6299db9ec823/xcb-0.8.2/build.rs:67:26

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I've tried adding dependencies to the travis file (here) based on some other issues and pull requests from this repo, but no luck. This is the first time I'm trying something like this so maybe I've made a silly error somewhere, let me know if it's unrelated and I'll close this issue.