💚 Update Travis CI's Doxygen.

Travis CI has Doxygen 1.7. We need 1.8 to generate our API
documentation. Get a static version and use it.
Always build API Documentation, but changed Doxygen configuration to
only print out errors in the documentation generation, so these can be
reviewed.

util/Doxyfile

@@ -23,3 +23,12 @@ ALIASES += "TODO=\todo"
 ALIASES += "BUG=\bug"
 ALIASES += "hooks=\xrefitem hooks \"Hooks\" \"Hooks List\""
 ALIASES += "HOOKS=\hooks"
+# Output
+QUIET = YES
+WARNINGS = YES
+# Dot tool config
+HAVE_DOT = YES
+DOT_IMAGE_FORMAT = svg
+INTERACTIVE_SVG = YES
+CLASS_GRAPH = YES
+COLLABORATION_GRAPH = NO