Remove dependency on `email_spec`
We depend on the matchers in the email_spec
gem, but this gem:
- is very out of date (last updated in 2014)
- does not support composable matchers, making some things awkward and in some cases inefficient (for example matching the body multiple times involves calling gsub on the body once for each iteration of the loop).
- the code in this module uses the old deprecated matcher syntax (see https://github.com/email-spec/email-spec/blob/master/lib/email_spec/matchers.rb). It would be easier to replace each matcher with a modern style matcher (only about 5 lines of code) in our code base rather than trying to maintain this library.