Please set up tests on Travis CI and test multiple versions of Ruby

[ndbroadbent]

I've been trying to help update this gem for Rails 5, but I can't get the tests to pass on my local machine. I'm trying to get them to run on Ruby 2.1.3. The json gem dependency also can't compile on Ruby 2.4.2, so it needs to be updated (part of the Rails 5 upgrade.)

The other useful thing about .travis.yml file is that it provides more information about how I can get the tests to work locally. Right now I don't know how to find which versions of Ruby are supported.

The maily_herald gem looks so awesome, and I can't wait to use it!