Pin Carer's completion rate module to 19 May

Instead of displaying data going back 9 weeks, just display back to the 19th of May because that's when we started collecting data with the `thank-you` label.

This change should be reverted in a few weeks once there is enough data in place to make a decent-looking graph. [A chore has been created for that](https://www.pivotaltracker.com/story/show/72963084).

You can see the problem with the graph here:

![screen shot 2014-06-10 at 14 39 29](https://cloud.githubusercontent.com/assets/121219/3230619/b5ddd80a-f0a4-11e3-96b0-8437c028046b.png)