[travis] Fix sourceforge certificate error
[travis] Fix sourceforge certificate error

[skip appveyor]

Use xcode8.3 image (macOS 10.12) to avoid sourceforge certificate error.
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