The previous code checks for Python 2, unconditionally, with
`has('python')`.  Unfortunately, this loads Python2 in vim, and then
prevents Python3 loading correctly.  As a result, if you specify you
want Python3 in your vimrc, and you have both Pythons installed into
Vim, then you'll get Python2.

Fix by checking the vimrc configuration before checking for the presence
of Python versions.