Upgrade to latest ruby
Drop Jeweler and use Bundler

This gem started before bundler was released, and at the time jeweler
was the hotness. It's not needed anymore.

Uses byebug instead of debugger for ruby 2.1

fixes: #50
Forces ssl for all connections

Soon, Harvest will no longer allow non-ssl connections to the API
Fixing the test suite

due_at isn't respected anymore, so passing in a string version for the
number of days after the issued_at

the of_user logic is a little faulty, so cleaning that up too
A better version of the refresh logic