benchmark:all becomes a performance spec

* Will stop deploy from falling over trying to load rack/test
* Need to run as `NUMBER_OF_RUNS=1000 bundle exec rspec spec/performance`
Merge pull request #101 from alphagov/move-benchmark-to-tests

benchmark:all becomes a performance spec
Update db/structure.sql

* Takes in the character differences in path/new_url from
  https://github.com/alphagov/transition/pull/444
* Knows about the UNIQUE INDEX on mappings (site_id, path)
Merge pull request #102 from alphagov/update-structure-sql

Update db/structure.sql