add $::osfamily == debian support
work around RubyGems 2.2.0 breaking ruby 1.8.7

Taken from https://github.com/freerange/mocha/commit/66bab2a8f4e7cd8734bf88e6f32157c0d5153125
Merge pull request #2 from jhoblitt/debian_support

add $::osfamily == debian support