Consider expanding ~ in kernel.json

I have following kernel.json:

```
{
"display_name": "Julia_test",
"language": "julia",
"argv": [
"julia",
"-i",
"-F",
"~/.julia/v0.3/IJulia/src/kernel.jl",
"{connection_file}"
],
"codemirror_mode":"julia"
}
```

which results in following error:

```
WARNING:root:kernel 1f12e33d-ba86-4aa4-bfb2-798de005e683 restarted
ERROR: could not open file /private/tmp/~/.julia/v0.3/IJulia/src/kernel.jl
in include at /usr/local/Cellar/julia/0.3.5_1/lib/julia/sys.dylib
in include_from_node1 at loading.jl:128
in process_options at /usr/local/Cellar/julia/0.3.5_1/lib/julia/sys.dylib
in _start at /usr/local/Cellar/julia/0.3.5_1/lib/julia/sys.dylib
```

It seems that `~` is not expanded, but IPython assumes that it is a relative path.