Add db upgrade logic for the fix introduced with the previous commit.

Fixes #711.

gourmet/plugins/nutritional_information/data_plugin.py

@@ -8,7 +8,7 @@
 class NutritionDataPlugin (DatabasePlugin):
 
     name = 'nutritondata'
-    version = 3
+    version = 4
 
     def setup_usda_weights_table (self):
         self.db.usda_weights_table = Table('usda_weights',self.db.metadata,
@@ -82,5 +82,7 @@ def update_version (self, gourmet_stored, plugin_stored, gourmet_current, plugin
             self.db.alter_table('nutritionaliases',self.setup_nutritionaliases_table,
                  {},['ingkey','ndbno','density_equivalent'])
 
-
-
+        if plugin_stored in ['1','2','3']:
+            # Set the length parameter of the ingkey and unit Strings to 255.
+            self.db.alter_table('nutritionconversions',self.setup_nutritionconversions_table,
+                 {},['id','factor'])