Support mail 2.7

[skalee]

Currently, gem requires "mail ~> 2.6.4", whereas the freshest mail gem release is 2.7.0, and there are RCs for 2.7.1. Unfortunately, some tests do fail when run against 2.7.0. It seems that it's due to bug in mail gem itself (which is fixed on current master). I don't think we need to support 2.7.0, but we should do 2.7.1 when it's released.

[skalee]

Mail 2.7.1 is out for quite a time, and we can add a != 2.7.0 dependency constraint on that gem. However, few other incompatibilities have shown up.