Print clippy version on Travis

[0xazure]

To help better debug issues like those we experienced in #24, we should print the versions of any extra tools we add to CI so we can compare version numbers and build hashes.

Elsewhere in the logs Travis prints out version numbers of the tools:

$ rustc --version
rustc 1.31.0-beta.5 (bf00632e3 2018-11-08)
$ rustup --version
rustup 1.14.0 (1e51b07cc 2018-10-04)
$ cargo --version
cargo 1.31.0-beta (efb7972a0 2018-10-24)

so we should too.

To do this for clippy, we need to run cargo clippy --version after a successful installation in our Travis config:

https://github.com/0xazure/supernova/blob/f4838f95e503f73bb8ed51abf790f66bb1febcf4/.travis.yml#L19-L20

so we can get something like:

$ cargo clippy --version
clippy 0.0.212 (125907ad 2018-09-17)

in the build log.

[biskit1]

Fixed with #39.