Fix db:seed:export task

* replace variable names with Hash to avoid invalid Ruby identifiers
* export all values as single-quoted to avoid string interpolation and
  treating BigDecimal numbers as Float
* #truncate table instead of #delete_all to avoid foreing_key
  constraints errors

Closes #56

db/seeds.rb

@@ -20,4 +20,6 @@ end
 # Formulas will be deleted as dependent on Quantities
 #[Source, Quantity, Unit].each { |model| model.defaults.delete_all }
 
-require 'seeds/units.rb'
+# To clear contents of the table, use #truncate instead of #delete_all. This
+# avoids foreign_key constraints errors.
+require_relative 'seeds/units.rb'