Rspec 3 Use `failure_message`

Deprecation Warnings:

```
`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /Users/mugi/git/paperclip/spec/support/matchers/have_column.rb:11:in `block in <top (required)>'.
```

spec/support/matchers/have_column.rb

@@ -8,7 +8,7 @@ RSpec::Matchers.define :have_column do |column_name|
     column && column.default.to_s == @default.to_s
   end
 
-  failure_message_for_should do |columns|
+  failure_message do |columns|
     "expected to find '#{column_name}', default '#{@default}' in #{columns.map{|column| [column.name, column.default] }}"
   end
 end