Update loofah for CVE-2019-15587.

Name: loofah
Version: 2.3.0
Advisory: CVE-2019-15587
Criticality: Unknown
URL: flavorjones/loofah#171
Title: Loofah XSS Vulnerability
Solution: upgrade to >= 2.3.1

Gemfile.lock

@@ -77,7 +77,7 @@ GEM
       contentful (~> 2.1)
       contentful-webhook-listener (~> 0.1)
       middleman-core (~> 3.4)
-    crass (1.0.4)
+    crass (1.0.5)
     diff-lcs (1.2.5)
     domain_name (0.5.20170404)
       unf (>= 0.0.5, < 1.0.0)
@@ -256,7 +256,7 @@ GEM
     listen (3.0.8)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
-    loofah (2.2.3)
+    loofah (2.3.1)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     lumberjack (1.0.12)