Update mail gem (~> 2.7.1) (#29443).

git-svn-id: http://svn.redmine.org/redmine/trunk@17595 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -12,7 +12,7 @@ gem "actionpack-xml_parser"
 gem "roadie-rails", "~> 1.3.0"
 gem "roadie", "~> 3.2.1"
 gem "mimemagic"
-gem "mail", "~> 2.6.4"
+gem "mail", "~> 2.7.1"
 gem "csv", "~> 1.0.2" if RUBY_VERSION >= "2.3"
 
 gem "nokogiri", "~> 1.8.0"