Update Nokogiri to protect against CVE-2019-5477

[lowellrex]

bundle exec rake security alerted us to a vulnerability in the Nokogiri library we use for XML and HTML parsing. This PR updates the library to a version that is not vulnerable to the disclosed CVE as per the directions presented by the maintainers (sparklemotion/nokogiri#1915).

$> bundle exec rake security
...
Updated ruby-advisory-db
ruby-advisory-db: 384 advisories
Looking for ~/Projects/caseflow/.security.yml
bundle-audit check --ignore=
Name: nokogiri
Version: 1.10.3
Advisory: CVE-2019-5477
Criticality: Unknown
URL: https://github.com/sparklemotion/nokogiri/issues/1915
Title: Nokogiri Command Injection Vulnerability via Nokogiri::CSS::Tokenizer#load_file
Solution: upgrade to >= 1.10.4

Vulnerabilities found!

Failed. Security vulnerabilities were found. Find the dependency in Gemfile.lock,
then specify a safe version of the dependency in the Gemfile (preferred) or
snooze the CVE in .security.yml for a week.

[codeclimate]

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