Update travis to run rustfmt against all crates

.travis.yml

@@ -22,7 +22,7 @@ before_script:
     - rustup target add armv7-unknown-linux-gnueabihf
     - if [[ $TRAVIS_RUST_VERSION == *"nightly"* ]]; then
          rustup component add rustfmt-preview;
-         cargo fmt --package=librespot-core -- --write-mode=diff;
+         cargo fmt --all -- --write-mode=diff;
       fi
 
 script: