add production.rb file

add production.rb file in preparation for Jenkins deployment.
remove :keep_releases setting from deploy.rb

Scot Dalton recommended removing the :keep_releases setting from deploy.rb
and relying on the NYU Libraries capistrano defaults instead.
set branch to master in deploy/production.rb

Per Scot Dalton recommendation, hardcode deployment branch to 'master'
because our assumption is that we will only ever deploy the master
branch to the production environment.
Merge pull request #51 from NYULibraries/feature/production-deploy

add production stage for deployment via capsitrano