Publish updated Ruby Gem or Update install commands to source GitHub

[embold-tyler]

The ruby gem version number looks the same, but trying to do gem 'solidus_easypost' on Solidus 4 outputs "Could not find compatible versions" and a whole bunch of errors. Looks like the recent changes to the gem in the repo have not been published with a new version number to rubygems.

Doing gem 'solidus_easypost', github: 'solidusio-contrib/solidus_easypost' works, so if that's recommended, the readme could just be updated to reflect it.