Updated Nokogiri to 1.6.0 - temporary hack of sax-machine dependency necessary

[sax-machine](https://github.com/pauldix/sax-machine) causes compatibility issues. This commit allows feedzirra to run Nokogiri 1.6.0 temporarily by putting the following in Gemfile:
```ruby
gem 'sax-machine', github: 'AutoUncle/sax-machine', branch: 'nokogiri_update', ref: '95e5f8fedb5ed2d1b3b6bdf3e9ac8c3dc5750de7'
gem 'feedzirra', github: 'AutoUncle/feedzirra', branch: 'nokogiri-update', ref: '9b0bd4dd69dc52a9db58dcf51f94107fb9233dad'
```

After a new version of sax-machine is released with [this pull request](https://github.com/pauldix/sax-machine/pull/45), [Gemfile](https://github.com/AutoUncle/feedzirra/compare/nokogiri-update?expand=1#diff-8b7db4d5cc4b8f6dc8feb7030baa2478R9) and [feedzirra.gemspec](https://github.com/AutoUncle/feedzirra/compare/nokogiri-update?expand=1#diff-6e2746e33d2df3d4e25f1802edb93b65R22) should be updated.