ipykernel installs invalid kernel.json

In #223 the setup.py learned to generate kernel.json, using `write_kernel_spec(overrides={'argv': make_ipkernel_cmd(executable='python')})`

This is completely invalid, as the default is to use sys.executable and hardcoding "python" when installing via:

- `python2 setup.py install` creates a kernel.json that refers to python3 on at least Arch Linux, where python is a symlink to python3
- `python3 setup.py install` creates a kernel.json that refers to python2 on on other operating systems/distributions where python is a symlink to python2

If it is very important to strip out the absolute leading path for sys.executable, then a better way to do that would be, in fact, to strip out the leading path using `os.path.split(sys.executable)[1]`