tweak request timeout settings for heroku

* use Rack::Timeout, set to 20s
* set unicorn timeout to 25s

Ideally these are set lower, around 10s/15s, however we need to reduce long-running requests (such as notifier batch pulls) before we can safely do this.
Merge pull request #68 from edx/feature/jsa/heroku-timeout

optimize request timeout settings for heroku