add attr_accessible :name that is used by seed for Spree::SotckLocation

Using a fresh set of migrations off of the master branch from `bundle exec rake railties:install:migrations` I got an error `Can't mass-assign protected attributes for Spree::StockLocation`. I simply added the `attr_accessible :name` and the migrations work succesfully.

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Output from rake db:migrate using jruby 1.7,3

    ==  CreateDefaultStock: migrating =============================================
    rake aborted!
    An error has occurred, all later migrations canceled:

    Can't mass-assign protected attributes for Spree::StockLocation: name
    path/db/migrate/20130814023858_create_default_stock.spree.rb:6:in `up'
    org/jruby/RubyBasicObject.java:1677:in `__send__'
    org/jruby/RubyKernel.java:2103:in `send'
    org/jruby/RubyArray.java:1613:in `each'
    org/jruby/RubyProc.java:249:in `call'
    org/jruby/RubyArray.java:1613:in `each'
    org/jruby/RubyArray.java:1613:in `each'
    org/jruby/RubyKernel.java:1046:in `load'
    Tasks: TOP => db:migrate
    (See full trace by running task with --trace)