Change register-kernel to manual kernel registration

[appleparan]

It fixes #1 and #2 .The actual cause of this bug is the incorrect assumption that jupyter must be installed in the global versions. So, instead of automatically updating the kernel in all existing virtualenvs, I changed it to manually register the kernel using register-kernel.

It might resolve https://github.com/aiguofer/pyenv-jupyter-kernel/pull/6, too.

Pros

• It fixes all problems in the issue.

Cons

• You need to execute pyenv register-kernel after pip install jupyter by yourself.