Update Rake task to only run on Sundays

Since the Rake task to populate the database will now be run
everyday by Heroku, create a version of the job that will only
run on Sundays to avoid using unneeded time on Heroku.

Additionally, this commit adds a few lines of code that populate
the "closed" column of the "pull_requests" table.
Reorganize file structure

Prior to this commit, both the public and views directories were
stored in the lib directory. This was because Sinatra looked for them
in a specific place relative to the server file.

Now they have been relocated and the path Sinatra will use to locate
them has been reconfigured.

Additionally, this commit adds stubbing to the repository spec that
prevents tests from failing due to exceeded API rate limits.