Fix schema number

Database code should always be in sync with the version
of the schema.  This is checked by ctkDICOMDatabaseTest3.

Closes #226

Libs/DICOM/Core/ctkDICOMDatabase.cpp

@@ -448,7 +448,7 @@ QString ctkDICOMDatabase::schemaVersion()
   //   so that the ctkDICOMDatabasePrivate::filenames method
   //   still works.
   //
-  return QString("0.5.1");
+  return QString("0.5.2");
 };
 
 //------------------------------------------------------------------------------