Fix escaping with recent libxml versions

Hi there - since recent versions of libxml-ruby now escape the node content, XML generated with Xbuilder now gets doubly escaped.  This pull request bumps the required version of libxml-ruby to 2.3.4 and drops Xbuilder's escaping method.

There was one failed test I wasn't sure about - https://github.com/nikitug/xbuilder/blob/a299f0b34c6ba5069cf2e85b5a6b0e2e3f086393/test/test_xbuilder.rb#L167-L170 checks that `xml.title("&amp;")` produces `"<title>&amp;</title>"`.  I think it ought to produce `"<title>&amp;amp;</title>"`, and have updated the test accordingly.