Many gem development updates + Better SMTP client support!

After finding that mailtrap did not work at all with ssmtp, a very simple SMTP
client, I took it upon myself to update this gem in order to support it.
I've merged the lsiden/pullreq branch into my own, and added some fixes
to mailtrap. 

 - Updated gem dev & test dependencies
 - Cleaned up many deprecation warnings
 - Cleaned up Rakefile
 - RSpec tests all PASSING!

 There is one unit test that did not pass for mailshovel,
and probably never did from what I can tell... 

    Mailshovel when sending a DELE command should delete messages on QUIT.
    test/mailshovel_test.rb:122

After messing with it for a while, I found that there is some problem
with the way the FakeTCP class rewinds the input for the DELE commands.
This results in only one of the mails being deleted.  I found very
erratic behavior when messing with the sleep amount and order of the
sleep & rewind in the say() function. Not sure how to fix that