Bump changelog version marker for release

CHANGELOG

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 0.5.0:
 
+0.5.0:
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 - Bug #17458 fixed. Normalize the :expires_in option to always be an integer even if the actual object passed in is a proxy to an integer, such as is the case with 2.hours from ActiveSupport which is actually an instance of ActiveSupport::Duration. Reported by [Steve Kickert steve@riverocktech.com]
 
 - Bug #19158 fixed. Don't prepend leading slash onto bucket name when deleting a bucket with the :force => true option.