Fix axis ticks on realtime sparklines.

* Rename confusing showEndTicks option to clearer showStartAndEndTicks.
* Pass showStartAndEndTicks to y-axis view.
* If showStartAndEndTicks is true, y-axis explicitly sets the tickValues
  to use on the axis, based on the maximum tick value calculated by us.
  D3 prioritises tickValues above any other tick options:
  https://github.com/mbostock/d3/wiki/SVG-Axes#wiki-tickValues

 Pivotal bug:
 https://www.pivotaltracker.com/s/projects/911874/stories/65885302

 Tests to follow.
Rename .tickValues to .tickValueList for clarity.

Currently we set a property called .tickValues on our D3 scale. This
is confusing, because there is an official D3 property called .tickValues
that is used on axes. There is no such property on D3 scales.

Rename this to .tickValueList for clarity: we are storing a list of
tick values, so this name is more descriptive and less likely to be
confused with the D3 axis property.
Add a new test for y-axis.

Test that y-axis passes the showStartAndEndTicks option, and that only start
and end ticks appear on the axis when it is true.
Merge pull request #249 from alphagov/bugfix/sparkline-axis-end-ticks

Fix bug where end ticks were not displaying for certain data values in sparklines.