Fixing test for Ubuntu OS version

facter variable name is wrong for Ubuntu. See https://github.com/spiette/puppet-ssh/issues/9.
Testing on an Ubuntu node with both facter 1.7.3 and 1.7.4, the variable name that holds this value is lsbmajdistrelease in both. Neither returned a value for operatingsystemmajrelease.
I'm now running a modified version of this module that uses lsbmajdistrelease in place of operatingsystemmajrelease for Ubuntu. I've submitted a pull request with this change, but I've only tested with these versions of facter, with Ubuntu 12.04, CentOS 6.5 and Debian 7.3.