add migration for creating entity ids for old entities

I reused the same semaphore name as the entity data migration for the entity id migration, as they shouldn't be running at the same time as each other, but it seems one of them is not releasing the semaphore fast enough. I'm wondering if the LUA release code is working as we expect. 

Currently flapjack exits on startup with:

2014-11-24T22:11:19+10:30 [FATAL] :: flapjack-processor :: Could not obtain lock for data migration. Ensure that no other flapjack processes are running that might be executing migrations, check logs for any exceptions, manually delete the 'entity_data_migration' key from your Flapjack Redis database and try running Flapjack again.
2014-11-24T22:11:19+10:30 [FATAL] :: flapjack-coordinator :: SystemExit
exit
/Users/jesse/src/flpjck/flapjack/lib/flapjack/data/migration.rb:77:in `exit'
/Users/jesse/src/flpjck/flapjack/lib/flapjack/data/migration.rb:77:in `migrate_entity_check_data_if_required'

Won't this line run even if semaphore is nil, when the exception is raised above? You could move the semaphore acquisition and error check/raise outside of the begin block, and leave the rest as is, I think.