Update Doxygen version

Owen · Owen · commit 349a04f1b60f · 2019-08-22T12:42:02.000+01:00
The old version is no longer available from this host.
diff --git a/.travis.yml b/.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:
