Update mail dependency of ActionMailer

For your consideration:

I'm working on a Rails application that requires a newer version of the **mail** gem (2.6). In order to use this version with Rails I needed to update **ActionMailer**'s dependency to allow for this version.

*  I've ran the specs against 2.6.1 with `bundle exec rake test` and have encountered no errors. 
*  I haven't deprecated any versions, so this should not introduce any backwards incompatibility. 

Thank you in advance.