Remove redundant exit handling

This is unnecessary with `set -e`
Remove symlinking to static

This is a throwback to when the admin tools shared assets from static.  This can't have been working for a long time (we now have multiple jenkins workers, and the static job isn't called that any more).
Run default rake task, not test.

This means we'll pick up all tasks that depend on default (including the check_for_bad_time_handling one).
Remove unneeded env variables.

* `DISPLAY` isn't needed since we switched from capybara-webkit to phantomjs
* Nothing in the app should depend on `FACTER_*` variables.
Merge pull request #196 from alphagov/cleanup_jenkins_sh

Cleanup jenkins.sh