Update doxygen version in CMake command.

Author: chrisdembia

.travis.yml

@@ -225,7 +225,7 @@ install:
 
   # Doxygen.
   - OSIM_CMAKE_ARGS+=(-DOPENSIM_DOXYGEN_USE_MATHJAX=off -DOPENSIM_SIMBODY_DOXYGEN_LOCATION="https://simtk.org/api_docs/simbody/latest/")
-  - if [[ "$DOXY" = "on" && "$TRAVIS_OS_NAME" = "linux" ]]; then OSIM_CMAKE_ARGS+=(-DDOXYGEN_EXECUTABLE=$HOME/doxygen/doxygen-1.8.10/bin/doxygen); fi
+  - if [[ "$DOXY" = "on" && "$TRAVIS_OS_NAME" = "linux" ]]; then OSIM_CMAKE_ARGS+=(-DDOXYGEN_EXECUTABLE=$HOME/doxygen/doxygen-1.8.14/bin/doxygen); fi
 
   - printf '%s\n' "${OSIM_CMAKE_ARGS[@]}"
   - cmake "${OSIM_CMAKE_ARGS[@]}"