enable rustfmt in travis ci

.travis.yml

 language: rust
 rust: stable
 cache: cargo
+before_script: (cargo install rustfmt || true)
 sudo: false
 git:
   depth: 1

ci/run-travis.sh

 # basic style check
 "$MY_PATH/check-basic-style.py"
 
+# rustfmt style check
+"$MY_PATH/rustfmt.sh"
+
 # check that everything compiles and all tests pass
 "$MY_PATH/test-rust.sh"