Fix/bug migrating single record

![](http://media.giphy.com/media/aAsOxi5ZVPpgA/giphy.gif)

## What? Why?

I ran into a bug when running Lhm.change_table against a table with only a single record.

In this case, both the `next_to_start` and the`limit` variables (in Chunker#execute) are the same thus never triggering the copy.

## What should be tested?

You can properly migrate tables with only 1 record using Lhm#change_table.

Note: I forked off off tag v2.1.0 (which is what [we](https://redbooth.com) are using right now). A bit busy to do it again for master but I can do so in a week or so.