Release 0.2.2

This release fixes a bug where setting the text of an element using
Oga::XML::Element#inner_text= would not set the parent element of the newly
created text node. This would result in the following:

some_element.inner_text = 'foo'

some_element.children[0].parent # => nil

Here parent is supposed to return some_element instead. See commit
142b467 for more information.