Fix sponsor logos

- make nilenso background medium
- add github back to associate sponsors
[sd] adding kitallis/steven speaker photos.
Merge branch 'master' of https://github.com/kaiwren/conference
[sd] changing to opaque background nilenso logo to satisfy saagers requirements.
[sd] merged.
Merge logo and speaker imange changes from Nilenso

- remove rubylearning duplicate
- rebalance logos