[travis] Avoid bug on osx related to RVM.
[travis] Avoid bug on osx related to RVM.

[skip appveyor]
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