Add version check for SafeBuffer vulnerability
user
```ruby
require 'brakeman/checks/base_check'

#Check for unsafe manipulation of strings
#Right now this is just a version check for
#https://groups.google.com/group/rubyonrails-security/browse_thread/thread/edd28f1e3d04e913?pli=1
class Brakeman::CheckSafeBufferManipulation < Brakeman::BaseCheck
  Brakeman::Checks.add self

  @description = "Check for Rails versions with SafeBuffer bug"

  def run_check

    if version_between? "3.0.0", "3.0.11"
      suggested_version = "3.0.12"
    elsif version_between? "3.1.0", "3.1.3"
      suggested_version = "3.1.4"
    elsif version_between? "3.2.0", "3.2.1"
      suggested_version = "3.2.2"
    else
      return
    end

    message = "Rails #{tracker.config[:rails_version]} has a vulnerabilty in SafeBuffer. Upgrade to #{suggested_version} or apply patches."

    warn :warning_type => "Cross Site Scripting",
      :message => message,
      :confidence => CONFIDENCE[:med],
      :file => gemfile_or_environment
  end
end
```
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