Make sure dates have a DST timezone set

Dates returned from GA need to be "decorated" with a DST timezone.

@pbadenski
Test timezone helper

@pbadenski
High level tests for DST conversion for responses

@pbadenski
Move 'timezone_helper' to datainsight_collector gem

@pbadenski
Use timezone helper from collector gem

@maxfliri
Merge pull request #5 from alphagov/timezone_fix

Make sure dates have a DST timezone set