Remove note about Gentoo ebuild

It is not hosted in this repo any longer.
Do not let powerline trigger loading wrong python

Used python version is controlled by `g:powerline_pycmd`. User configuration now
has top priority: if `g:powerline_pyeval` is set powerline will not try to use
`pyeval()` emulation in old Vim versions.

Closes #937 as WONTFIX
Merge pull request #938 from ZyX-I/noloadpython

Do not let powerline trigger loading wrong python
Add information about g:powerline_pycmd to error message
Merge branch 'update-powerline.vim-error' into develop