Update elasticsearch download source

elasticsearch started hosting their own downloads:

http://www.elasticsearch.org/blog/2012/12/17/new-download-service.html
Move web interface downloads from Github to s3/graylog2-releases

Github no longer supports downloads.

Generalized S3 url from:

https://groups.google.com/forum/#!searchin/graylog2/s3/graylog2/QecD9cnR1gg/ZKysq8gM3SAJ
Default web interface version to 0.9.6.p2