Barrier mesh issue with latest fmesher

See this issue: https://github.com/eliaskrainski/INLAspacetime/issues/3

The error is 

```
Error in mesh2fem.barrier(mesh, triBarrier): fm_manifold(mesh, c("S", "R")) is not TRUE
```

In the meantime, installing a version of fmesher < 0.1.6 should fix it.