Deprecation Warning for Ruby 1.9.2

[nacengineer]

Bruce,

I get a deprecation warning for line 39 of bitmask_attribute.rb

current line is
model.bitmasks[attribute] = returning HashWithIndifferentAccess.new do |mapping|

for deprecation fix it needs to be:
model.bitmasks[attribute] = ( HashWithIndifferentAccess.new ).tap do |mapping|

Thanks for the great plugin!
Dave