Fixing error message concatenation for an attribute when there is only 1...

... error message
-----------------------------------
Currently when there's 1 error message per attribute, the :join is being invoked on a string trying to concatenate the error messages:
ActionView::TemplateError (undefined method `join' for "is not a number":String)
I.e.:
```
From: /Users/nz/.rvm/gems/ruby-1.8.7-p374@newui/bundler/gems/twitter_bootstrap_form_for-f5e5ef6273f9/lib/twitter_bootstrap_form_for/form_builder.rb @ line 231 TwitterBootstrapFormFor::FormControls#errors_for:
```
```
    230: def errors_for(attribute)
 => 231:   require 'pry'; binding.pry
    232:   self.object.errors[attribute].try(:join, ', ')
    233: end

[1] pry(#<TwitterBootstrapFormFor::FormControls>)> self.object.errors[attribute]
=> "is not a number"
```
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This PR fixes that allowing 1 or more error messages per attribute.