Fix race condition when setting mutex key

[clupprich]

There's the possibility of a race condition when we're acquiring a new mutex:

DistributedMutex.attempt_to_lock first checks if current_lock is nil and only then sets the mutex key. However, the command that's used there is SET, which

If key already holds a value, it is overwritten[…].

We need to either use SETNX (which is deprecated) or use SET command with the NX option to

Only set the key if it does not already exist.

I'm also setting the expiration time for the key in the same command, as we don't wanna touch that if it already exists.

Had to limit the version of Rake that can be used, as it would error with newer versions.

[clupprich]

Pinging @ericescalante who originally implemented the switch from memcached -> Redis in a58f6ca.