support chef 12

[chantra]

Chef 12 verifies the sha256sum of the downloaded file.
https://github.com/opscode/chef/blob/12-stable/lib/chef/provider/file.rb#L346

https://github.com/poise/python/blob/master/attributes/default.rb#L38
should read
default['python']['checksum'] = '7f49c0a6705ad89d925181e27d0aaa025ee4731ce0de64776c722216c3e66c42'

[chantra]

PR #133 would fix this

[Noah Kantrowitz]

Fixed by #133