Add performance testing dir to .ackrc

This file has hundreds of thousands of lines and is not useful
to be included when using ack.
begin removing non flattened code
Revert "begin removing non flattened code"

This reverts commit 1d07c47fd06ed3481b8451d6b02cd46c5409817b.
Merge pull request #372 from alphagov/add-test-perf-dir-to-ackrc

Add performance testing dir to .ackrc