Delete session records from the database if they're over 25 hours old.

- This sets a schedule using the Whenever gem to run this rake task every day at 3am. 3am was chosen as an arbitrary time. If anyone has any other suggestions or times which they deem more sensible, do say!
- Twenty-five hours old was the suggestion in Transition's Pivotal story, so I used that.
- I haven't yet tested the scheduler as that would involve waiting until 3am, but the rake task itself works - the sessions table in my dev VM now has one row as opposed to the > 44000 it had before.
- This is done because the production database had tens of thousands of rows too and we don't need them all there after a day.