try to use doxygen in system

.travis.yml

@@ -4,9 +4,9 @@ env:
   global:
     - LANG="zh_CN.UTF-8"
     - CMAKE_PREFIX=$HOME/prebuilt/cmake ;
-    - CMAKE_LINUX_URL=https://cmake.org/files/v3.10/cmake-3.10.2-Linux-x86_64.sh ;
+    - CMAKE_LINUX_URL=https://cmake.org/files/v3.11/cmake-3.11.0-Linux-x86_64.sh ;
     - PANDOC_PREFIX=$HOME/prebuilt/pandoc ;
-    - PANDOC_LINUX_URL=https://github.com/jgm/pandoc/releases/download/2.1.1/pandoc-2.1.1-linux.tar.gz
+    - PANDOC_LINUX_URL=https://github.com/jgm/pandoc/releases/download/2.1.3/pandoc-2.1.3-linux.tar.gz
     - DOXYGEN_PREFIX=$HOME/prebuilt/doxygen
     - DOXYGEN_LINUX_URL=http://ftp.stack.nl/pub/users/dimitri/doxygen-1.8.14.linux.bin.tar.gz
 matrix:
@@ -59,19 +59,19 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-      env: USE_CC=doxygen USE_CXX=doxygen PANDOC_BIN=$PANDOC_PREFIX/bin/pandoc DOXYGEN_BIN=$DOXYGEN_PREFIX/bin/doxygen
+      env: USE_CC=doxygen USE_CXX=doxygen PANDOC_BIN=$PANDOC_PREFIX/bin/pandoc DOXYGEN_BIN=doxygen # DOXYGEN_BIN=$DOXYGEN_PREFIX/bin/doxygen
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-6.0
+            # - llvm-toolchain-trusty-6.0
           packages:
             - graphviz
-            - libclang-dev
-            - libclang-6.0-dev
+            - doxygen
       script:
+        - sudo apt-get install -f
         - cd "docs";
-        - if [ ! -e "$DOXYGEN_BIN" ]; then
+        - if [ "doxygen" != "$DOXYGEN_BIN" ] && [ ! -e "$DOXYGEN_BIN" ]; then
 
             mkdir -p "$DOXYGEN_PREFIX";