Raise unicorn memory limits

The base memory usage of a Unicorn worker is about 500-600 MB now. The previous memory limits (min: 400, max: 600) were too low because Unicorn Worker Killer picks a random number between 400 and 600 MB and will kill the process if it happens to fall in that number. As a result, there was a high probability that a Unicorn worker would be killed after every request.

To fix this, we raise the limits to match GitLab.com: 1 GB and 1.28 GB.

This is the peak memory usage of GitLab over the past week:

Closes #4279 (closed)