Update travis to use doxygen 1.8.19 (#28)

Use newer version of doxygen for better support of markdown files.

.travis.yml

@@ -1,20 +1,23 @@
-sudo: false
-
+before_install:
+ - cd ${TMPDIR-/tmp}
+ - wget -q http://doxygen.nl/files/doxygen-1.8.19.src.tar.gz
+ - tar -xzvf doxygen-1.8.19.src.tar.gz
+ - mkdir doxygen_build
+ - cd doxygen_build
+ - cmake ../doxygen-1.8.19/
+ - make
+ - export PATH="${TMPDIR-/tmp}/doxygen_build/bin:$PATH"
+ - cd ${TRAVIS_BUILD_DIR}
+ 
 branches:
   only:
     - master
 
-addons:
-  apt:
-    packages:
-      - doxygen
-
 script:
-  - doxygen Doxyfile
+  - doxygen ./docs/Doxyfile
 
 deploy:
   provider: pages
-  skip_cleanup: true
   local_dir: docs/html
   github_token: $GH_REPO_TOKEN
   on: