Update to Savon gem to fix entity escaping issues

We ran into an issue where Savon was incorrectly parsing the SOAP response from Primo by decoding escaped entities incorrectly.

This fix updates Savon, which removes the need for the extra escaping done in lib/exlibris/primo/web_service/response/base.rb.  It also removes the need for using inner_html instead of inner_text to read attributes.  Since &lt; symbols are now properly escaped, inner_text returns the expected value, while inner_html will actually escape the less than and greater than symbols.

We noticed the problem on MLA records from Primo Central, because the MLA source comes through with an image tag that was escaped properly by Primo, but modified in the body by Savon so the raw xml given to nokogiri looked like this: `<source>MLA International Bibliography<img src="http://exlibris-pub.s3.amazonaws.com/mlalogo_001.jpg" style="vertical-align:middle;margin-left:7px"></source>`

Nokogiri interpreted <img as a new element, which ended up breaking the nesting and xpath parsing.