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/templates/units.erb

@@ -1,9 +1,9 @@
 Unit.transaction do
-  Unit.defaults.delete_all
+  ActiveRecord::Base.connection.truncate(Unit.table_name)
 <% Unit.defaults.ordered.each do |unit| %>
 <%= "\n" if unit.base.nil? %>
-  unit_<%= unit.symbol %> =
-    Unit.create symbol: "<%= unit.symbol %>",<% unless unit.base.nil? %> base: unit_<%= unit.base.symbol %>, multiplier: <%= unit.multiplier.to_scientific %>,<% end %>
-                description: "<%= unit.description %>"
+  units['<%= unit.symbol %>'] =
+    Unit.create symbol: '<%= unit.symbol %>',<% unless unit.base.nil? %> base: units['<%= unit.base.symbol %>'], multiplier: '<%= unit.multiplier.to_scientific %>',<% end %>
+                description: '<%= unit.description %>'
 <% end %>
 end