Commit cee110b2 (IPython Notebook: Fixing coding crimes for section
names) changed IPython notebook section name to use '-' as word
delimiter but forgot to update users, and this way wendelin build
started to fail:

    INFO While:
    INFO   Installing.
    INFO   Getting section ipython_notebook.
    INFO Error: The referenced section, 'ipython_notebook', was not defined.

Fix it. (And I've made sure with whole-tree git grep that there is no
more ipython notebook users except wendelin in whole slapos.git so far)

/reported-by @Tyagov
/cc @tiwariayush
/reviewed-by TrustMe

lab.nexedi.com is now our main domain for git hosting.
lab.nexedi.cn continues to work for backward-compatibility reasons.

/reviewed-by: TrustMe

/reviewed-by: TrustMe

This reverts commit 8dac4823.

Let's try buildint again after hopefully fixing rubygems issue in
1f9adc65 (*: Pin rubygems library version when used; Do pinning by url)

/cc @klaus, @Tyagov

Reviewed-by: TrustMe

This reverts commit bf75e8a9.