BLD: Add doxygen to install

Author: Jack Underwood

.travis.yml

@@ -21,7 +21,7 @@ before_install:
     python-all-dev zlib1g-dev qtbase5-dev qtdeclarative5-dev libqt5opengl5-dev
     qttools5-dev qttools5-dev-tools libgraphicsmagick++1-dev
     libopenscenegraph-dev libpoppler-dev libcairo2-dev libwpg-dev libmspub-dev
-    libcdr-dev libvisio-dev
+    libcdr-dev libvisio-dev doxygen
 
 script:
   - |