Remove aws-ses gem and replace AWS::SES exceptions

https://www.agileplannerapp.com/boards/173808/cards/4274

we've switched to sending emails through SMTP
instead of the SES HTTP API.

note about exceptions: if the recipient of an
email is blacklisted by SES, an SMTP 554 error
is returned. the corresponding exception raised
in ruby code is:

Net::SMTPFatalError
http://ruby-doc.org/stdlib-1.9.3/libdoc/net/smtp/rdoc/Net/SMTPFatalError.html
Merge pull request #230 from alphagov/remove-aws-ses-gem

Remove aws-ses gem and replace AWS::SES exceptions