Bump multi_xml to fix CVE: 2013-0175

bundler-audit output:

Name: multi_xml
Version: 0.4.4
CVE: 2013-0175
Criticality: Unknown
URL: http://osvdb.org/show/osvdb/89148
Title: multi_xml Gem for Ruby XML Parameter Parsing Remote Command Execution
Solution: upgrade to >= 0.5.2