add info about v0.1.5 require Rails 3

[ruckus-matte]

I tried upgrading to 0.1.5 in my 2.3 app and received a NoMethodError 'unscoped' error.

unscoped was introduced in Rails 3.

v0.1.4 list.rb line 213

acts_as_list_class.find(:first, :conditions => conditions, :order => "#{position_column} DESC")

v0.1.5 list.rb line 225

acts_as_list_class.unscoped.find(:first, :conditions => conditions, :order => "#{position_column} DESC")

It should probably be noted in the README that 0.1.5 require Rails 3 and up.

(e)

[swanandp]

Aye, the 0.1.5 push happened just this weekend. I'll update the README, thanks for reporting!

[swanandp]

Updated the README, I think we are good here.