Replace Sham Rack with WebMock

* Use WebMock to disable HTTP requests to external services in the test
  suite.
* Move `spec/spec_helper.rb` logic to its own template to improve
  readability.

WebMock has broader community support, is well-maintained, and works
with a variety of HTTP libraries.

https://github.com/bblimke/webmock
Move SimpleCov to fix load order