Update videos on landing page

Hello,

The four current videos are from 2014 and ready for a refresh. I suggest we pick four out of the following videos:

1. Mike Mintz - Rendering ReQL directly in the browser using react-rethinkdb https://youtu.be/WchhRxdNojk
2. Slava Akhmechet - What's coming up for RethinkDB in 2016 https://youtu.be/zL5_EsF06DM
3. Travell Perkins - Experience APIs: A RethinkDB Business Use Case https://youtu.be/rm2zerSz6aE
4. William Stein - Using RethinkDB in production at SageMathCloud https://youtu.be/WU6eSckPR7E
5. Peter Hamilton - Robust Changefeeds with Elixir and RethinkDB https://youtu.be/oG_KK3tbBdE

[dalanmiller]

I think Elixir while cool might be a bit fringe-y for the messaging we want to be putting here on our homepage. Instead #5 I propose one these:

Rob Conery @ DevDay2015 in PL => https://www.youtube.com/watch?v=Ee1v_SuECRk

Highlights ease of use and RethinkDB developer friendliness and is an excellent excellent intro. (Just the sound of his voice will reduce your blood pressure.)

Realtime Stories:

These would be awesome to show as actual businesses using RethinkDB for great success. I like these two the most, and I think Platzi in #3rd place.

Lendio => https://www.youtube.com/watch?v=D4uvpzxUwqI
NextGxDx => https://www.youtube.com/watch?v=KYH8kn5sIEU