Speed up Whitehall mappings import

Until now, we have been loading the host for each old URL in the CSV from the
database each time. This manually caches the hosts.

In development this changed the runtime for:
➜  transition git:(master) ✗ time FILENAME=tmp/1396018394-whitehall_mappings.csv be rake import:whitehall:mappings

from:
FILENAME=tmp/1396018394-whitehall_mappings.csv bundle exec rake   181.65s user 112.01s system 88% cpu 5:31.37 total

to:
FILENAME=tmp/1396018394-whitehall_mappings.csv bundle exec rake   95.10s user 54.83s system 86% cpu 2:53.45 total
Merge pull request #234 from alphagov/speed_up_whitehall_import

Speed up Whitehall mappings import by 50%