Remove custom backpressure support in favor of RxJava 1.1's built-in …

[JakeWharton]

…buffer latest.

[f2prateek]

LGTM!

Given that this is a built in RxJava operator now, does it even make sense for us to do it by default? Clients could easily opt in to this behaviour.

[JakeWharton]

There's little benefit, I suppose. It's nice when backpressure Just Works™, but there is non-zero overhead (although it is very close to zero).

[JakeWharton]

It can be advantageous to have backpressure supported higher up in the chain to avoid downstream work when it isn't needed (i.e., when back pressure is being applied). In this case we defer calling get() which will do value lookup and potential conversion. Very small win in this library's instance, but you can see how in other circumstances it's good to have higher up (e.g., if conversion needed to do a API call, for example).