Force loofah 2.3 in response to CVE-2018-8048

- The report actually says:
"loofah gem 2.13.0 is vulnerable (CVE-2018-8048). Upgrade to 2.2.1."

- On closer inspection, you'll see that 2.2.1 is a *downgrade* from 2.13.0
A known issue:
flavorjones/loofah#209

Gemfile.lock

@@ -276,7 +276,7 @@ GEM
     kaminari-mongoid (1.0.1)
       kaminari-core (~> 1.0)
       mongoid
-    loofah (2.13.0)
+    loofah (2.3.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     mail (2.7.1)