Run luacheck in travis

.travis.yml

@@ -20,6 +20,20 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-7
 
+    - name: "Builtin Luacheck"
+      language: generic
+      compiler: null
+      os: linux
+      sudo: false
+      addons:
+        apt:
+          packages:
+          - luarocks
+      before_install:
+        - luarocks install --local luacheck
+      script:
+        - $HOME/.luarocks/bin/luacheck builtin
+
     - env: CLANG_TIDY=clang-tidy-7
       compiler: clang
       os: linux
@@ -100,4 +114,3 @@ matrix:
           sources: &sources
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-7
-