Fix bug in flat data for completion_rate module

This completion_rate does not support the case where multiple pieces
of data are used in the compilation of a single data point representing
a given time period.

Additionally, this module would sometimes result in duplicated output
in the table that accompanies a graph, for example on 'SORN':

    1 to 30 November 2013                    123%
    1 to 30 November 2013                    123%
Merge pull request #852 from alphagov/bugfix-completion-rate-flat-data

Fix bug in flat data for completion_rate module