fix issues from recent upgrades

[lostapathy]

• Mail 2.7.0 seems to have some bugs in unicode handling that were not present in 2.6.x, and are fixed in the 2.7.1-rc releases. This causes some emails to fail parse and thus prevents ticket generation. I have confirmed that reverting to 2.6.6 fixes the issue and that 2.7.1.rc1 fixes it as well. I have been unable to reduce this down to a simple test case to add to the suite, unfortunately.

• Ticket's belongs_to :user should not have been made optional during the rails 5.1 upgrade, as user_id is required.

[coveralls]

Coverage remained the same at 92.382% when pulling 6dbbc374d6c99fb88db61db3525dd57b45e9d1fb on lostapathy:rails_5_followup into fb6fc674574353ba3609bf2ce0975031fdcf5a5b on ivaldi:master.

[frenkel]

Great, thanks!