Disable URL caching to always get the latest data from GitHub (comments, statuses)

[czechboy0]

The default NSURLSession's configuration respects GitHub's response TTL, which is up to 60 seconds. However, if we want to sync more often (GitHub's authenticated rate limit is 5000 requests per hour, which is plenty to make Buildasaur very responsive to the team's workflow, when the sync interval is lower, cca 15sec), currently we get the same, cached response for ~4 more syncs, leading to duplicate comments etc. We need to disable the cache to always get the uncached state of the PR (status and comments, most importantly)

[czechboy0]

Fixed in 23d13de