upgrade mail gem

Heroku was nice to say:

> You are receiving this notification because you have at least one app running a version of the Mail gem with a known memory regression. Upgrading can reduce memory footprint in a basic application by up to 36%.
> http://www.schneems.com/2014/11/07/i-ram-what-i-ram.html
> To fix, please upgrade to mail 2.6.3 above.
