Fix test failures due to bad time handling check
Make jenkins.sh run the default rake task.

This was only running the 'test' taks, and therefore not running the bad
time handling check.
Add a git clean to jenkins.sh

This ensures we're running with a clean state.  It also prevents
logfiles from gorwing without bounds on the CI machines
Merge pull request #46 from alphagov/fix_time_handling

Fix time handling