Add capnp as required dependencies (#1)
Add capnp as required dependencies (#1)

capnp should be specified to be installed, otherwise building fails with:

```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Failed, description: "Error while trying to execute `capnp compile`: Failed: No such file or directory (os error 2).  Please verify that version 0.5.2 or higher of the capnp executable is installed on your system. See https://capnproto.org/install.html" }', ../src/libcore/result.rs:788
note: Run with `RUST_BACKTRACE=1` for a backtrace.

```
pipedown/noise,pipedown/noise