Due to an impedance mismatch between PostgreSQL, the C langauge and Haskell's
Persistent library, 'Word64' values larger than the maxiumum value of a signed
64 bit integer come back as a 'PersistRational' with a denominator of '1'. We
have to handle that case.

Closes: #334

This is a workaround fix for:

   #351

This fix will not require the database to be re-synced, but will result
in some invalid values for the 'fixed_cost' field of the 'pool_update'
table being display as negative value.

The constraint will be droppped only if it exists (eg someone had previously
dropped in manually).