Set the libxml2 default back to 2.9.10
Set the libxml2 default back to 2.9.10

2.9.12 is pretty broken right now and it looks like they may have an
  upstream release to resolve some problems.

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

chef/omnibus-software,mattray/omnibus-software,chef/omnibus-software,chef/omnibus-software,chef/omnibus-software,mattray/omnibus-software,chef/omnibus-software,mattray/omnibus-software,mattray/omnibus-software,mattray/omnibus-software,chef/omnibus-software