canvas.raster with linear upsampling bug

It seems that linear upsampling using the canvas.raster method can under some conditions cause an error in numba as shown [here](https://travis-ci.org/ioam/holoviews/jobs/292018082#L2205).

```
File "/home/travis/miniconda/envs/test-environment/lib/python3.5/site-packages/numba/parfor.py", line 191, in run
self.func_ir.blocks = simplify_CFG(self.func_ir.blocks)
File "/home/travis/miniconda/envs/test-environment/lib/python3.5/site-packages/numba/ir_utils.py", line 986, in simplify_CFG
block = blocks[p]
KeyError: 'Failed at nopython (convert to parfors)\n582'
```

This is likely something to do with ``prange`` optimization in numba and may already have been patched in latest numba but I have not had time to investigate.