Fix deprecation warning

To quote Julia:

```
WARNING: writesto(cmd,args...) is deprecated, use open(cmd,"w",args...) instead.
 in plot at /media/ryan/stuff/.julia/v0.4/Graphs/src/dot.jl:76
 in include at ./boot.jl:242
 in include_from_node1 at loading.jl:128
 in process_options at ./client.jl:300
 in _start at ./client.jl:382
 in _start_3B_4033 at /media/ryan/stuff/julia/usr/bin/../lib/julia/sys.so
```