Modules shouldn't use Nokogiri

Nokogiri has a habit of shipping vulnerable builds of libxml2. For
example, see this:

http://www.ubuntu.com/usn/usn-1904-1/

and compare to Nokogiri's bundled requirements:

https://github.com/sparklemotion/nokogiri/blob/master/dependencies.yml

While Nokogiri is quite pleasant to use, it really shouldn't be trusted
to handle potentially malicious data. Imagine if a "vulnerable" target
was actually a malicious honeypot, lying in wait for a poor Metasploit
user to come along and parse out its payload. (OT: does such a thing
have a clever name? If not, I propose "beehive" to imply the offensive
capabilities of such a honeypot.)

Nokogiri is used elsewhere in Metasploit, but those functions handle
data sourced from the Metasploit user herself, so those XML hunks are
nominally trustworthy.
Land #2935 - msftidy for nokogiri