travis: Disable doc generation

Travis doesn't have sphinx installed by default and it's also the wrong
version for our documentation

common/travis/configure.sh

@@ -29,4 +29,5 @@ cmake \
   -DCMAKE_CXX_FLAGS="${CXXFLAGS}" \
   -DCMAKE_BUILD_TYPE="${BUILD_TYPE}" \
   -DBUILD_TESTS:BOOL="${BUILD_TESTS:-OFF}" \
+  -DBUILD_DOC:BOOL="${BUILD_DOC:-OFF}" \
   "${FLAGS[@]}" ..