Clean up dictionary temp file after use.
Correct comment about the amount of entropy.

I must have been sleepy or distracted: the calculations for the rate
of collision were correct, but I indicated that there were 32 bits of
entropy rather than 28.
Automatically caffeinate some long-running tasks.

OS X's `caffeinate` allows you to prevent the system from going to sleep
until a given command is complete. This is very useful for potentially
long-running processes like provisioning a vagrant host.

In particular, we will caffeinate git repacks and vagrant up, down and
provision operations.