Date range for kpi covers a year not 13 months.

For example, if the data is:

"_timestamp": "2013-01-01T00:00:00+00:00"
"end_at": "2014-01-01T00:00:00+00:00"

it should render:

Jan 2013 to Dec 2013

not

Jan 2013 to Jan 2014

Updated tests to cover this.
Merge pull request #467 from alphagov/bugfix/kpi-date-range

Date range for kpi covers a year not 13 months.