Fix time-related failure in state pension calculator

Fix time at post-April and update values for context block
Use timecop to set the date on tests failing

These tests test a bit of the SA that uses Date.today. If we don't
timecop this, we'll have to update these tests weekly as they work off a
date in the future that's calculated.
Timecop the mat-pat calculator tests
Restructure timecop tests for maternity pay
Update calculator tests with new rating value
Merge pull request #293 from alphagov/time-related-test-fixes

Time related test fixes