Properly activate shoulda in minitest

the default integration in this version of shoulda doesnt
inject itself into minitest test classes
Fix date calculation bugs

The friendly_time_diff method was doing its calculations using DateTime
values rather than dates. The DateTimes sometimes have different
timezone values (depending on how they were generated), or may be a mix
of timezone aware and timezone-unaware date classe.

I've re-implemented the friendly_time_diff method in a way which uses
Dates for the calculations. Dates are not sensitive to timezone issues.
The friendly_time_diff method actually only calculates differences at a
date-granularity so this works better.
Fix intermittent errors due to time zone issues

irb(main):001:0> Time.parse('2012-04-06 14:53:54')
=> 2012-04-06 14:53:54 +0100

which does not equal 20120406T145354Z

Use Time.zone.parse and an explicit time zone to avoid this.
Bug fixes for state pension calculator WIP
Complete state pension calculator copy changes
Merge branch 'fixing-date-bugs' into state_pension_calculator_fixes
Merge pull request #872 from alphagov/state_pension_calculator_fixes

State pension calculator fixes