wrap two scopes in procs to fix migration/startup issues

[ahamid]

Two class_evald bitmask_attributes scopes are not wrapped in procs, which results in migration failures as they attempt to interact with missing tables:

/home/aaron/.rvm/gems/ruby-1.9.3-p0@menagerie-pm-3.1/gems/activerecord-3.1.3/lib/active_record/connection_adapters/postgresql_adapter.rb:1021:in `async_exec': PGError: ERROR:  relation "users" does not exist (ActiveRecord::StatementInvalid)
LINE 4:              WHERE a.attrelid = '"users"'::regclass
                                        ^
:             SELECT a.attname, format_type(a.atttypid, a.atttypmod), d.adsrc, a.attnotnull
              FROM pg_attribute a LEFT JOIN pg_attrdef d
                ON a.attrelid = d.adrelid AND a.attnum = d.adnum
             WHERE a.attrelid = '"users"'::regclass
               AND a.attnum > 0 AND NOT a.attisdropped
             ORDER BY a.attnum

The lines in question are:

https://github.com/joelmoss/bitmask_attributes/blob/master/lib/bitmask_attributes/definition.rb#L131
https://github.com/joelmoss/bitmask_attributes/blob/master/lib/bitmask_attributes/definition.rb#L149

This patch wraps those in procs which allows db migrations to run.
Tests still pass.

[joelmoss]

cheers!