add CI for formatting

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index a7cf8a3..e1a5b82 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,16 @@
 
 matrix:
   include:
+    # clang-format specific job
+    - os: linux
+      sudo: false
+      env: CLANG_FORMAT
+      addons:
+        apt:
+          sources: [ 'ubuntu-toolchain-r-test' ]
+          packages: [ 'libstdc++6', 'libstdc++-5-dev' ]
+      script:
+        - make format
     - os: linux
       sudo: false
       env: CXX=g++-5