travis: Add apidoc check

This requires at least Ubuntu 14.04 (the Doxygen version in 12.04 has some
issues with our Doxyfile and prints lots of warnings).

.travis.yml

@@ -41,6 +41,8 @@ matrix:
   include:
     - compiler: gcc
       env: TEST=dist
+    - compiler: gcc
+      env: TEST=apidoc
     - compiler: gcc
       env: TEST=win64 MONOLITHIC=yes
     - compiler: gcc