[travis] Don't build _opensim completely alone.
[travis] Don't build _opensim completely alone.

_opensim can be built while the tests and examples are built.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core