Update diffy gem to 3.1.0

ChangeLog: https://github.com/samg/diffy/blob/master/CHANGELOG

Gemfile

@@ -117,7 +117,7 @@ gem 'truncato',           '~> 0.7.8'
 gem 'nokogiri', '~> 1.6.7', '>= 1.6.7.2'
 # Diffs
-gem 'diffy', '~> 3.0.3'
+gem 'diffy', '~> 3.1.0'
 # Application server
 group :unicorn do

Gemfile.lock

@@ -180,7 +180,7 @@ GEM
       railties
       rotp (~> 2.0)
     diff-lcs (1.2.5)
-    diffy (3.0.7)
+    diffy (3.1.0)
     docile (1.1.5)
     doorkeeper (4.2.0)
       railties (>= 4.2)
@@ -847,7 +847,7 @@ DEPENDENCIES
   default_value_for (~> 3.0.0)
   devise (~> 4.2)
   devise-two-factor (~> 3.0.0)
-  diffy (~> 3.0.3)
+  diffy (~> 3.1.0)
   doorkeeper (~> 4.2.0)
   dropzonejs-rails (~> 0.7.1)
   email_reply_parser (~> 0.5.8)