bump mail gem to 2.6.4

closes CNVS-34065

test plan:
- do something that sends an email
- it should work

Change-Id: I21c40bd414bd43580faf7423a829fca85c5625ad
Reviewed-on: https://gerrit.instructure.com/95345
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Pedro Fajardo <pfajardo@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Tested-by: Jenkins

Gemfile.d/app.rb

@@ -70,9 +70,7 @@ gem 'jwt', '1.2.1', require: false
 gem 'json-jwt', '1.6.5', require: false
 gem 'twilio-ruby', '4.2.1'
 
-gem 'mail', '2.5.4', require: false
-  gem 'treetop', '1.4.15', require: false
-    gem 'polyglot', '0.3.5', require: false
+gem 'mail', '2.6.4', require: false
 gem 'marginalia', '1.4.0', require: false
 gem 'mime-types', '1.25.1', require: 'mime/types'
 gem 'mini_magick', '4.2.7'

gems/incoming_mail_processor/incoming_mail_processor.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "activesupport", ">= 3.2", "< 5.1"
 
-  spec.add_dependency "mail", "2.5.4"
+  spec.add_dependency "mail", "2.6.4"
   spec.add_dependency "html_text_helper"
   spec.add_dependency "utf8_cleaner"
   spec.add_dependency "canvas_statsd"