Make equivalent-xml work in spec_helper

For ruby 2.1, to avoid errors like:

Failure/Error: expect(@dsdoc.ng_xml).to be_equivalent_to <<-EOF
expected <?xml version="1.0"?>
  <mods xmlns="http://www.loc.gov/mods/v3">
  <subject><topic>foo</topic></subject><subject><topic>bar</topic></subject><subject><topic>quux</topic>
  </subject></mods>
to respond to `equivalent_to?`