install kernelspec in wheels

[minrk]

• wheel kernelspec relies on PATH env to find python, which should be okay when installed to sys.prefix, as opposed to system-wide.
• can't use universal wheels anymore, since Python version is in kernelspec name and display name

It would be ideal to get a hardcoded path at install time (pypa/pip#4032), but for envs this should do until that becomes possible.

After this, it should be a lot harder for the native kernelspec to not be installed, which would be ideal. Plus, uninstalling ipykernel will also uninstall the kernelspec.

[takluyver]

I'm not convinced by relying on PATH - that assumes that people always activate a virtualenv rather than launching scripts inside it directly.

[minrk]

We could handle the executable python in jupyter_client to explicitly mean sys.executable (jupyter/jupyter_client#235), which should take care of that.

[minrk]

Well, crap. I swear I didn't click the merge button, but GitHub interpreted a click in the comment area as a click of the merge button (not sure if it's GitHub's or my browser's fault). I can revert and re-issue the PR if you want, or we can continue to review this PR and I'll open a new one for any changes that should result.

I cleaned up travis.yml a bit to do a 'regular' install rather than an editable one for testing.

[takluyver]

Let's carry on without reverting.

[cben]

Nice!
It seems this installs the kernel.json pip install . but not on pip install -e . (which README.md recommends for developement). EDIT: might be related to pypa/sampleproject#30 and/or pypa/pip#3160