Travis CI: Build before formatting check

cargo fmt does not build the code, and running it in a fresh clone of
the codebase will fail because the protobuf code has not been generated.

.travis.yml

@@ -8,5 +8,6 @@ before_script:
   - rustup component add rustfmt
 
 script:
+  - cargo build --verbose --release --all
   - cargo fmt --all -- --check
   - cargo test --verbose --release --all