Update API to reflect clearer function identifier

![getinsertpic.com](http://media2.giphy.com/media/FVBLiFGpKEgKs/200.gif)
*^ my response to bad function names*

## Problem

The `sequence` function does not accurately reflect what it does, which in this instance is to validate that the provided message is sequential before attempting to update the "last seen" number and proceeding to write the renderered content.

## Solution

Rename the function as seen in this PR.

> Note: this PR requires a change to the `alephant-publisher-queue` https://github.com/BBC-News/alephant-publisher-queue/pull/4