Update Doxygen version

The old version is no longer available from this host

.travis.yml

@@ -49,7 +49,7 @@ jobs:
     - stage: Linter + Doxygen + non-debug Ubuntu/gcc-5 test
       env:
         NAME: "DOXYGEN-CHECK"
-        DOXYGEN_VERSION: "1.8.14"
+        DOXYGEN_VERSION: "1.8.16"
       addons:
         apt:
           sources:

scripts/run_doxygen.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 # Check doxygen version
-EXPECTED_VERSION="1.8.14"
+EXPECTED_VERSION="1.8.16"
 doxygen --version | grep -x $EXPECTED_VERSION > /dev/null
 if [ $? -ne 0 ]
 then